The engine oil Bibles - everything you need to know about engine oil including viscosity, friction reducers, additives, oil types, sludge, SAE and API classifications and ratings, what all the codes and markings mean, how your engine oil works, how to keep your engine running at peak fitness and much more.

The Engine Oil Bible


How much do you value the engine in your car? The life of your engine depends in no small part on the quality of the oil you put in it - oil is its lifeblood. People typically don't pay much attention to their oil - oil is oil, right? In the bad old days, maybe, but engine oil underwent something of a revolution in the 80's and 90's when hot hatches, 16-valve engines and turbos started to become popular. High compression engines and black death meant the days of one oil catering for everyone were over.
Take Castrol for example. They led the field for years with their GTX mineral oil. This was eventually surpassed by semi-synthetic and fully synthetic oils, including GTX2 and GTX3 Lightec. Those were surpassed by Formula SLX and most recently, Castrol GTX Magnatec. All manufacturers have a similar broad spectrum of oils now. I just mention Castrol in particular as they're my oil of choice.

What does my oil actually do?

Engine oil performs many functions. It stops all the metal surfaces in the engine from grinding together and tearing themselves apart from friction, and it transfers heat away from the combustion cycle. Engine oil also holds in suspension all the nasty by-products of combustion like silica (silicon oxide) and acids. Finally, engine oil minimises the exposure to oxygen and thus oxidation at higher temperatures. It does all of these things under tremendous heat and pressure.

How do I read the '5W40' type number?

As oils heat up, they generally get thinner. Single grade oils get too thin when hot for most modern engines which is where multigrade oil comes in. The idea is simple - use science and physics to prevent the base oil from getting too thin when it gets hot. The number before the 'W' is the 'cold' viscosity rating of the oil, and the number after the 'W' is the 'hot' viscosity rating. So a 5W40 oil is one that behaves like a 5-rated single grade oil when cold, but doesn't thin any more than a 40-rated single grade oil when hot. The lower the 'winter' number (hence the 'W'), the easier the engine will turn over when starting in cold climates. There's more detail on this later in the page under both viscosity, and SAE ratings.

A quick guide to the different grades of oil.

Fully SyntheticCharacteristics
Fuel economy savings
Enhances engine performance and power
Ensures engine is protected from wear and deposit build-up
Ensures good cold starting and quick circulation in freezing temperatures
Gets to moving parts of the engine quickly
Better protection
Good protection within the first 10 minutes after starting out
Roughly three times better at reducing engine wear
Increased oil change intervals - don't need to change it quite so often
Basic protection for a variety of engines
Oil needs to be changed more often

What the heck was 'Black Death'?

'Black Death' first appeared in the early 80's when a sticky black substance was found to be the cause of many engine seizures in Europe. It was extremely frustrating for vehicle owners because dealers and mechanics had no idea what was going on. Black Death wasn't covered under insurance - owners had to pay to fix engines themselves. Many engines were affected but Ford and Vauxhall (GM) suffered the most. Faster roads, higher under-hood temperatures, tighter engineering tolerances and overworked engine oils turned out to be contributors to the problem. The oils just couldn't handle the demands, and changed their chemical makeup under pressure into a sort of tar-like glue. This blocked the oil channels in the engines, starving them of lubrication and causing them to seize.
I don't recommend this but you can reproduce the effect with a frying pan, cooking oil and a blowtorch. The cooking oil will heat up far quicker than it is designed to, and will turn to a sticky black tar in the pan. Either that or it will set fire to your kitchen. Which is why I said "don't do this".
Anyway, burning kitchens aside, Black Death was the catalyst for the production of newer higher quality oils, many of them man-made rather than mineral-based.

Black death for the 21st century: sludge

Black Death is still with us today but it has a new name - sludge. The cause is the same as Black Death and it seems to be regardless of maintenance or mileage. The chemical compounds in engine oils break down over time due to prolonged exposure to high temperatures and poor maintenance habits. When the oil oxidises, the additives separate from it and begin to chemically break down and solidify, leading to the baked-on oil deposits turning gelatinous, like black yoghurt. What doesn't help is that due to packaging, modern engines have smaller sumps than their older counterparts, and so hold less oil. This lower volume of oil can't hold as much combustion by-product in suspension, and that can lead to earlier chemical breakdown.
The most common factor in sludge buildup is a combination of mineral oils, a lack of maintenance by the car owner and harsh driving conditions. However, a 2005 Consumer Reports article discovered that some engines from Audi, Chrysler, Saab, Toyota, and Volkswagen appear prone to sludge almost no matter how often the oil is changed.

What does sludge look like?

Engine oil sludge Engine oil sludge was contacted by a BMW driver who had been having a particularly harsh time with sludge and was discussing it on the Bimmerfest forums. He posted some images of his problem and other readers posted similarly-framed images of the same engine components in "normal" condition. Here are two of those photos. On the left is what the cam case should look like in a well maintained engine when photographed through the oil filler cap. On the right is what the same type of engine looks like when suffering sludge buildup.
In this example, the consensus was that the sludge buildup was caused by an overheating engine, oil that hadn't been changed for 20,000 miles of stop-go city driving, a lot of cold starts and a period of about 12 months in storage without an oil change.

Picture credit: Ketchup at the Bimmerfest forums

Curing sludge

There are no hard and fast rules for curing an engine of sludge buildup. If it's really bad, flushing the engine might be the only cure, but that could cause even more problems. If flushing the engine results in bits of sludge getting lodged where they can do more damage, the engine is actually worse off.
It's interesting to note that some race techs have reported sludge buildup in race engines as a result of aftermarket additives being used in conjunction with the regular oil. The chemical composition of the additives isn't as neutral as some companies would like us to believe, and combined with particular types of oil and high-stress driving, they can cause oil breakdown and sludge to appear. The lesson from them appears to be "don't use additives".

When is sludge not sludge?

combustion leak tester Easy; when it's an oil and water emulsion from a leaking or blown head gasket. If this happens, there will be a white-cream coloured gel on the inside of the oil filler cap that looks like vanilla yoghurt or mayonnaise. The cap is typically cooler than the rest of the cam cover and so the oil/water mix tends to condense there. If the underside of the filler cap has this sort of deposit on it, chances are the engine has a blown head gasket. A surefire way to confirm this is if the oil level is going up and the coolant level is going down. The coolant gets through the breaks in the head gasket and mixes with the oil. When it gets to the sump it separates out and the oil floats on top.
A more accurate way to check for this condition is to use a combustion leak tester, or block tester. In America, NAPA sell them for about $45 (part #BK 7001006). In England, Sealey sell them for about £70 (model number VS0061). Combustion leak testers are basically a turkey baster filled with PH liquid, with a non-return valve at the bottom. To use one, run the engine for a few minutes until it is warm (not hot) then turn it off. Use a protective glove (like an oven glove) and take the radiator or reservoir cap off. Plug the bottom of the combustion leak tester into the hole and squeeze the rubber bulb on top. It will suck air from the top of the coolant through the non-return valve and bubble it through the PH liquid. If the liquid changes colour (normally blue to yellow), it means there is combustion gas in the coolant which means a head gasket leak.


There is one other possible cause for the mayonnaise: a blocked scavenger hose. Most engines have a hose that comes off the cam cover and returns to the engine block somewhere via a vacuum line. This is the scavenger hose that scavenges oil vapour and gasses that build up in the cam cover. If it becomes blocked, it can result in a buildup of condensation inside the cam cover, which can manifest itself as the yellow goop inside the filler cap.

VW / Audi sludge problems

The 1.8T engines in the older Audi A4's, Audi TT, VW Passat, Jetta, Golf, New Bettle, are all very prone to sludge build-up. VW/Audi recommends 10,000 mile service intervals, but oil changes can be done between "services", and should be done if the vehicle is driven in heavy traffic, offroad, and non-highway use. VW/Audi only warrant an engine if the customer has proof of all their oil changes. Since 2004, VAG recommended all their 1.8T engines use synthetic oil.
So if you own one of these sludge-prone engines, what can you do? Obviously, Volkswagen Audi Group (VAG) states that you use only VW/Audi recommended oil. You should also keep up on your oil changes, making them more frequent if you drive hard or haul a lot of cargo. The most important thing for the VW or Audi owner is this: if the oil light comes on and beeps the high pitch beep that almost everyone ignores, pull over and shut the engine down immediately. Many VAG engines can be saved by this procedure. Have the vehicled towed to a VAG dealer. Their standard procedure is to inspect the cam bearings; if they're not scored, the oil pan will be removed and cleaned out and all the crankcase breather hoses and the oil pickup tube will be replaced. They'll do an oil pressure test with a mechanical gauge, and hopefully will also replace the turbo lines. Finally, the turbo will be checked for bearing free-play. The VAG turbos run really hot even with proper oil and coolant supply - that's why you need a good quality synthetic in them.

Toyota sludge problems

For their part, Toyota have the dubious honour of having the most complaints about sludge buildup in their engines - over 5,000 in 2008 alone. At the time of writing there is a class action suit going on against them. Details can be found at

Saab sludge problems

For an example of sludge in a Saab 9 5 Aero with only 42,000 miles on it, you might be interested to read my case study on this engine, put together with the help of a reader. Our sludge case study.

Like the site? The page you're reading is free, but if you like what you see and feel you've learned something, a small donation to help pay down my car loan would be appreciated. Thank you.

Mineral or synthetic motor oil?

Mineral oils are based on oil that comes from dear old Mother Earth which has been refined. Synthetic oils are mostly concocted by chemists wearing white lab coats in oil company laboratories. The only other type is semi-synthetic, sometimes called premium, which is a blend of the two. It is safe to mix the different types, but it's wiser to switch completely to a new type rather than mixing.


Despite their name, most synthetic derived motor oils (ie Mobil 1, Castrol Formula RS etc) are actually derived from mineral oils - they are mostly Polyalphaolifins and these come from the purest part of the mineral oil refraction process; the gas. PAO oils will mix with normal mineral oils which means they can be added to mineral oils without causing an engine to seize up (although it is rumoured that Mobil 1 is actually made by reformulating ethanol).
These bases are pretty stable, meaning 'less likely to react adversely with other compounds' because they tend not to contain reactive carbon atoms. Reactive carbon has a tendency to combine with oxygen creating an acid. (In an oil this would be A Bad Thing.) They also have high viscosity indices and high temperature oxidative stability. Typically a small amount of diester synthetic (a compound containing two ester groups) is added to counteract seal swell too. These diesters act as a detergent and will attack carbon residuals. So think of synthetic oils as custom-built oils. They're designed to do the job efficiently but without any of the excess baggage that can accompany mineral based oils.

Pure synthetics

Pure synthetic oils (polyalkyleneglycol) are the types used almost exclusively within the industrial sector in polyglycol oils for heavily loaded gearboxes. These are typically concocted by even more intelligent scientists in even whiter lab coats, who break apart the molecules that make up a variety of substances, like vegetable and animal oils, and then recombine the individual atoms that make up those molecules to build new, synthetic molecules. This process allows the chemists to actually "fine tune" the molecules as they build them. Clever stuff. But Polyglycols don't mix with normal mineral oils.

[amsoil] While we're on synthetic oils, we should mention Amsoil. They contacted and asked us to point out the following:
Amsoil do NOT produce or market oil additives and do not wish to be associated with oil additives. They are a formulator of synthetic lubricants for automotive and industrial applications and have been in business for 30+ years. They are not a half-hour infomercial or fly-by-night product, nor have they ever been involved in a legal suit regarding their product claims in that 30+ year span. Many Amsoil products are API certified, and ALL of our products meet and in most cases exceed the specifications of ILSAC, AGMA etc. Their lubricants also exceed manufacturers specifications and Amsoil are on many manufacturers approval lists. They base their claims on ASTM certified tests and are very open to anyone, with nothing to hide.

Amsoil recommend engine oil additives are NOT to be used with their products. They have a pretty good FAQ on the Amsoil website: Amsoil FAQ (external link). There is also a particularly good page talking about testing Amsoil in taxis.

Will fully synthetic oil in an older engine cause the seals to leak?

This question comes up a lot from people who've just bought a used vehicle and are wanting to start their history with the car on fresh oil.
The short answer: generally speaking, not any more. The caveat is that the engine must be in good working order and not be leaking already. If that's the case, most modern oils are fully compatible with the elastomeric materials that engine seals are made from, and there shouldn't be any issues with leaks.
The longer answer:

Mixing Mineral and Synthetic oils

Here's the current thinking on the subject of mixing mineral and synthetic oils. This information is based on the answer to a technical question posed on the Shell Oil website:
There is no scientific data to support the idea that mixing mineral and synthetic oils will damage an engine. When switching from a mineral oil to a synthetic, or vice versa, there is the potential to leave a small amount of residual oil in the engine. That's perfectly okay because synthetic oil and mineral-based motor oil are, for the most part, compatible with each other. (The exception is pure synthetics. Polyglycols don't mix with normal mineral oils.)
There is also no problem with switching back and forth between synthetic and mineral based oils. In fact, people who are "in the know" and who operate engines in areas where temperature fluctuations can be especially extreme, switch from mineral oil to synthetic oil for the colder months. They then switch back to mineral oil during the warmer months.
There was a time, years ago, when switching between synthetic oils and mineral oils was not recommended if one product or the other had been used for a long period of time. People experienced problems with seals leaking and high oil consumption but changes in additive chemistry and seal material have taken care of those issues. And that's an important caveat. New seal technology is great, but if you're still driving around in a car from the 80's with its original seals, then this argument becomes a bit of a moot point - the seals are still going to be subject to the old leakage problems no matter what newfangled additives the oil companies are putting in their products.

Flushing oils

These are special compound oils that are very, very thin. They almost have the consistency of tap water both when cold and hot. Typically they are 0W/20 oils. Their purpose is for cleaning out all the gunk which builds up inside an engine.


Some hybrid vehicles now require 0W20, so if you're a hybrid driver, check your owner's manual. Honda switched to recommending 0W20 in 2011 to meet their CAFE ratings (thinner oil gives less drag on engine parts which improves - fractionally - the mpg). If the figures for the 2010 models are compared to their 2011 counterparts, cars like the Element and CR-V received a tiny mpg boost in the official figures despite being the exact same car. They achieved this by remapping the gearbox shift points and dropping the cold viscosity rating on the oil. In reality, unless you live in northern Alaska, or do an above average number of cold-start short journeys, 5W20 ought to be more than suitable.

Do I need an engine flush?

Unless there's something seriously wrong with an engine, like it's been filled it with milk or shampoo, there is not normally a need for an engine flush. If you do decide to do an oil flush, there's two ways of doing it. You can either use a dedicated flushing oil, or a flushing additive in your existing oil. Either way it's wise to change the filter first so you have a clean one to collect all the gunk. (This typically means draining the oil or working fast). Once you have a new filter in place, and the flushing oil (or flushing solution) in there, run the engine at a fast idle for about 20 minutes. Finally, drain all this off (and marvel at the crap that comes out with it), replace the oil filter again, refill with a good synthetic oil and voila! Clean(er) engine. For the curious amongst you, looking in the oil filter that was attached when you did the flush will be an educational exercise in the sort of debris that used to be in your engine.
CAVEAT. There's a condition attached when using flushing oils. In an old engine you really don't want to remove all the deposits. Some of these deposits help seal rings, lifters and even some of the flanges between the heads, covers, pan and the block, where the gaskets are thin. I have heard of engines with over 280,000km that worked fine, but when flushed, failed in a month because the blow-by past the scraper ring (now really clean) contaminated the oil and ruined the rod bearings.
Another thing to consider: if the flush is not done thoroughly, the gunk that it dislodges can remain in the engine and get lodged somewhere else doing serious damage. For example by blocking oilways and starving other engine components. With modern engines, if you do regular oil changes, most oils have good detergent packages in the mix that mean you shouldn't ever need an engine flush.
Finally - major manufacturers including Ford, GM, Nissan, Toyota and others have all issued service bulletins recommending against engine flushes. In most cases if you suffer an engine failure or problem after a flush, it won't be covered by your manufacturer warranty, and good luck getting the lube place to admit it was their fault.
Buyer beware. For the curious, the author has never had an oil flush done on any engine.

Using Diesel oil for flushing

A question came up some time ago about using diesel-rated oils to flush out petrol engines. The idea was that because of the higher detergent levels in diesel engine oil, it might be a good cleaner / flusher for a non-diesel engine. Whilst most of the diesel oil specification oils can be used in old petrol engines for cleaning, a low specification oil should be used to ensure that the engine is not over-cleaned, which can cause a loss of compression (for example). Generally speaking, an SAE 15W/40 diesel engine oil for about 500 miles might do the trick.

Which oil should you buy? (the short version)

That all depends on the car, the owner's pocketbook and how the car is intended to be driven and serviced. All brands claim theirs offers the best protection available - until they launch a superior alternative. It's like washing powders - whiter than white until new Super-Nukem-Dazzo comes out. For most motorists and most cars, a quality mainstream oil is the best, like Castrol GTX. Moving up a step, you could look at Duckhams QXR, Castrol Protection Plus and GTX3 Lightec. The latter two of these are designed specifically for engines with catalytic converters. They're also a good choice for GTi's and turbo engines. Go up a step again and you're looking at synthetic oils aimed squarely at the performance market like Mobil-1.
To help owners navigate through the maze of oils available, there's a site available (the motor oil evaluator) that aims to lessen the confusion with a relatively balanced scoring system based on published specifications such as viscosity and pour point. It's a good starting point for more in-depth info.

Which oil should you buy? (the long version)

Quality Counts! It doesn't matter what sort of fancy marketing goes into an engine oil, or how many naked babes smear it all over their bodies, or how bright and colourful the packaging is, it's what's written on the packaging that counts. Specifications and approvals are everything. There are two established testing bodies. The API (American Petroleum Institute), and the European counterpart, the ACEA (Association des Constructeurs Europeens d'Automobiles - replaced CCMC in 1996). Most people will never have heard of either of them, but their stamp of approval will be seen on the side of every reputable can of engine oil.


The API The API classifications are different for petrol and diesel engines: For petrol, listings start with 'S' (meaning Service category, but it can also be thought of as Spark-plug ignition), followed by another code to denote standard. 'SN' is the current top grade but 'SH' is still the most popular.
For diesel oils, the first letter is 'C' (meaning Commercial category, but it can also be thought of as Compression ignition). 'CJ' is the highest grade at the moment, (technically CJ-4 for heavy-duty) but 'CH' is the most popular and is well adequate for passenger vehicle applications.


Castrol recently upgraded all their oils and for some reason, Castrol diesels now use the 'S' rating, thus completely negating the little aid-memoire above. So the older CC,CD,CE and CF ratings no longer exist, but have been replaced by an 'SH' grade diesel oil. This link is a service bulletin from Castrol, explaining the situation.


The CCMC/ACEA The ACEA standards are prefixed with an 'A' for petrol engines, 'B' for passenger car diesel, 'C' for diesel with particulate filter, or 'E' for heavy-duty diesel. (The older CCMC specifications were G,D and PD respectively). The ACEA grades may also be followed by the year of issue which will be either '04 or '07 (current). Coupled with this are numerous approvals by car manufacturers which many oil containers sport with pride.

The full ACEA specs are:

† Not suitable for all engines - should ONLY be used in engines specifying this fuel efficient grade. Refer to the manufacturer handbook of contact your local dealer if you're not sure.
Mineral oils:

Part / full synthetic oils:

Low SAPS diesel (Sulphated Ash, Phosphorous, Sulphur):
For diesel engines fitted with a diesel particulate filter (DPF) - a filter unit in the exhaust that takes out the microscopic soot particles. Regular diesel oils used in engines that have a DPF can cause the filter to become blocked with ash.

Many OEM are now using their own specifications to capture these specifications. eg. Mercedes 229.31/51, BMW Longlife 04, VW 507 00 etc.
There is also a trend now towards manufacturers requiring their own specifications - in this case the OEM specification is the one that needs to be adhered to. If it says BMW Longlife 04, the oil must say this on the pack to be suitable for use.

Typically, these markings will be found in a statement similar to: Meets the requirements of API SH/CD along the label somewhere. The API Service Symbol ought to still be somewhere on the packaging:


Beware the fake API symbol

fake api symbolfake API symbolSome unscrupulous manufacturers (and there's not many left that do this) will put a symbol on their packaging designed to look like the API symbol without actually being the API symbol. They do this in an effort to pump up the 'quality' of their product by relying on people not really knowing exactly what the proper API symbol should look like. To the left is an example of a fake symbol - it looks similar but as long as you remember what to look for, you won't get taken by this scam.
Amsoil are one of the biggest inadvertent offenders of the fake API symbol. Take a look at one of their labels here on the right. See that little starburst that says "Fuel efficient formula SL-CF"? It's actually not an API-certified SL or CF oil. (To be fair, some Amsoil products are API certified and they do have the correct labelling, but their top-tier products do not). The issue of their lack of API certification on these products caused such a stir at Amsoil that they had to generate a FAQ to answer the most commonly-asked questions. A copy of that can be found here: Amsoil & API Licensing. It explains everything logcially and clearly, and it's not scientific doublespeak. Which is nice.

A Brief History of API ratings
Some people have asked about the old standards, and although they're not especially relevant, some rampant plagiarism from an API service bulletin means we can bring you all the API ratings right back from when the earth was cooling. expand/contract the table below to see the ratings.

Petrol Engines Diesel Engines
Category Status Service Category Status Service
SN Current For all automotive engines presently in use. Introduced in the API service symbol in November 2010 CJ-4 Current Introduced in 2006 for high-speed four-stroke engines. Designed to meet 2007 on-highway exhaust emission standards. CJ-4 oils are compounded for use in all applications with diesel fuels ranging in sulphur content up to 500ppm (0.05% by weight). However, use of these oils with greater than 15ppm sulfur fuel may impact exhaust aftertreatment system durability and/or oil drain intervals. CJ-4 oils are effective at sustaining emission control system durability where particulate filters and other advanced aftertreatment systems are used. CJ-4 oils exceed the performance criteria of CF-4, CG-4, CH-4 and CI-4.
SM Current For all automotive engines presently in use. Introduced in the API service symbol in November 2004 CI-4 Current Introduced in 2002 for high-speed four-stroke engines. Designed to meet 2004 exhaust emission standards implemented in 2002. CI-4 oils are formulated to sustain engine durability where exhaust gas recirculation (EGR) is used and are intented for use with diesel fuels ranging in sulphur content up to 0.5% weight. Can be used in place of CD, CE, CF-4, CG-4 and CH-4
SL Still current but nearly obsolete For all automotive engines presently in use. Introduced in the API service symbol in 1998 CH-4 Current Introduced in 1998 for high-speed four-stroke engines. CH-4 oils are specifically designed for use with diesel fuels ranging in sulphur content up to 0.5% weight. Can be used in place of CD, CE, CF-4 and CG-4.
SJ Still current but nearly obsolete For all automotive engines presently in use. Introduced in the API service symbol in 1996 CG-4 Current Introduced in 1995 for high-speed four-stroke engines. CG-4 oils are specifically designed for use with diesel fuels ranging in sulphur content less than 0.5% weight. CG-4 oil needs to be used for engines meeting 1994 emission standards. Can be used in place of CD, CE and CF-4.
SH Obsolete For model year 1996 and older engines. CF-4 Current Introduced in 1990 for high-speed four-stroke naturally aspirated and turbo engines. Can be used in place of CD and CE.
SG Obsolete For model year 1993 and older engines. CF-2 Current Introduced in 1994 for severe duty, two stroke motorcycle engines. Can be used in place of CD-II.
SF Obsolete For model year 1988 and older engines. CF Current Introduced in 1994 for off-road, indirect-injected and other diesel engines including those using fuel over0.5% weight sulphur. Can be used in place of CD.
SE Obsolete For model year 1979 and older engines. CE Obsolete Introduced in 1987 for high-speed four-stroke naturally aspirated and turbo engines. Can be used in place of CC and CD.
SD Obsolete For model year 1971 and older engines. CD-II Obsolete Introduced in 1987 for two-stroke motorcycle engines.
SC Obsolete For model year 1967 and older engines. CD Obsolete Introduced in 1955 for certain naturally aspirated and turbo engines.
SB Obsolete For older engines. Use this only when specifically recommended by the manufacturer. CC Obsolete Introduced in 1961 for all diesels.
SA Obsolete For much older engines with no performance requirement. Use this only when specifically recommended by the manufacturer. CB Obsolete Introduced in 1949 for moderate-duty engines.
CA Obsolete Introduced in 1940 for light-duty engines.

[sae] Grade counts too!The API/ACEA ratings only refer to an oil's quality. For grade, the place to look is the SAE (Society of Automotive Engineers) ratings. These describe the oil's function and viscosity standard. Viscosity means the substance and clinging properties of the lubricant. When cold, oil can become like treacle so it is important that any lube is kept as thin as possible. Its cold performance is denoted by the letter 'W', meaning 'winter'. At the other end of the scale, a scorching hot oil can be as thin as water and about as useful too. So it needs to be as thick as possible when warm. Thin when cold but thick when warm? That's where MultiGrade oil comes in. For ages, good old 20W/50 was the oil to have. But as engines progressed and tolerances decreased, a lighter, thinner oil was required, especially when cold. Thus 15W/50, 15W/40 and even 15W/30 oils are now commonplace.

The question of phosphorus and zinc.

Phosphorus (a component of ZDDP - Zinc Dialkyl-Dithio-Phosphate) is the key component for valve train protection in an engine and 1600ppm (parts per million) used to be the standard for phosphorus in engine oil. In 1996 the EPA forced that to be dropped to 800ppm and then more recently (2004?) to 400ppm - a quarter of the original spec. Valvetrains and their components are not especially cheap to replace and this drop in phosphorus content has been a problem for many engines (especially those with flat-tappet type cams). So why was the level dropped? Money. Next to lead, it's the second most destructive substance to shove through a catalytic converter. The US government mandated a 150,000 mile liftime on catalytic converters and the quickest way to do that was to drop phosphorous levels and bugger the valvetrain problem. Literally.
In the US, Mobil 1 originally came out with the 0W40 as a 'European Formula' as it was always above 1000 ppm. This initially got them out of the 1996 800ppm jam and knowledgeable consumers sought it out for obvious reasons. Their 15W50 has also maintained a very high level of phosphorus and all of the extended life Mobil synthetics now have at least 1000ppm. How do they get away with this? They're not classified as energy/fuel conserving oils and thus do not interfere with the precious government CAFE (corporate average fuel economy) ratings. (See my section on the EPA and fuel economy in the Fuel and Engine Bible for more info on this). This also means that they don't get the coveted ratings of other oils but they do protect your valvetrain. The same rule of thumb is true for racing oils like Royal Purple - because they're not classified as energy / fuel conserving, it would seem they still contain good quantities of ZDDP.
Royal Purple is a popular oil for Mustang enthusiasts, as it's formulated for performance vehicles that are looking to maximize results at the track or drag strip.
In fact, as a general rule-of-thumb, staying away from XX-30 oils and going to 10W-40 or higher might be the way to go if you have an older engine. 10W-40 and above is generally also not considered to be 'gas saving' and like the Mobil example above, doesn't mess with the CAFE rating.
If you live in England, Castrol market a product with ZDDP in the product description - 'Castrol Classic Oil With ZDDP Anti-Wear Additive' although it's not mainstream enough to be available everywhere. You'll have to find a specialist dealer. Castrol Classics. In the US, Rislone manufacture an oil supplement to boost the ZDDP content of your existing oil. Rislone Engine Oil Supplement.

API rating backward compatibility and 2V engines

This section contains information from Bruce Dance, Brian over at and LN Engineering and their combined experience with API ratings and 2 valve engines

If you own a two-valve spark ignition engine or certain diesel engines (which do not have to meet recent emission standards) the only sensible (ie widely available) oil to put in right now is synthetic or semisynthetic to meet API SL/CF and not a higher rating. As I touched upon above, oils with a CG and higher rating typically don't contain enough ZDDP, and the replacement friction modifiers don't work in highly loaded valve trains (generally older engines especially those with 2V design). If you try to compensate by adding a ZDDP additive into a newer oil it still might not work because of interactions with other additives in the oil.
Why the discrepancy in the ratings? The API no longer include a valve train wear test that accurately simulates 2V cam follower loading. They do perform a test that simulates 4V loading and then they allow a lot of wear to occur and still 'pass'. The ACEA tests are a lot tougher but still not tough enough. Whilst the newer CG, CH and higher API oil standards should be 'better in every way', they are really just 'improved in some ways'. Hence the increasing use of manufacturer-specific standards.
There is a lot of info kicking around on the web on this topic because it has caused a LOT of problems with some engines especially Porsche aircooled units.
One of my readers found out when he went to buy oil for his (modern 4V common rail diesel) Nissan that they expressly prohibit the use of CG or higher rated oils. Nissan mandate that owners use CF oils in these engines. It's worth noting that the CF spec was already out of date when these engines were built but Nissan did not use the latest API spec because it wasn't good enough!
The fact that API have dropped the CF tests/standard does not in any way improve the later oils that do not meet this standard.

Marine Diesels and other special considerations.

Inland Marine Diesels (and certain road vehicles under special conditions) can (and do) glaze their bores due the low cylinder wall temperatures causing the oil (and more importantly the additive pack) to undergo a chemical change to a varnish-like substance. The low temperature is caused by operating under light load for long periods.
This is related to engine design, some engines being nearly immune to it and others susceptible. The old Sherpa van diesel engines were notorious for this problem. The "cure" (such as it is) is to use a low API specification oil, such as CC. Certain engine manufacturers/marinisers are now marketing the API CC oil for this purpose under their own name (and at a premium). There are some modern engines where the industrial/vehicle manual states API CF and the marinised manual states API CC/CD. {Thanks to Tony Brooks for this information.}

Marine Oils.

The question can be asked; "why are marine engine oils so expensive and why can't regular motor oil be used in a marine engine instead?". The National Marine Manufacturers Association Oil Certification Committee (click here for more info) introduced a four-stroke engine oil test and standard called the 4T certification. This specification is meant to assist boaters and manufacturers in identifying four-stroke cycle engine oils that have been specially formulated to withstand the rigors of marine engine operation. The certification was prompted by the growing influence of four-stroke engines in the marine market and their unique lubrication demands. So the simple answer is that regular road-based engine oil products don't contain rust inhibitors and won't pass the 4T certification. Lakes, waterways and the sea are a lot more aggressive an environment for an engine to operate around than on land.
Note : the NMMA have long had a similar specification for 2-stroke oils destined for marine use, called the TC-W3® certification.

The eBay problem

This paragraph may seem a little out of place but I have had a lot of problems with a couple of eBay members (megamanuals and lowhondaprelude) stealing my work, turning it into PDF files and selling it on eBay. Generally, idiots like this do a copy/paste job so they won't notice this paragraph here. If you're reading this and you bought this page anywhere other than from my website at, then you have a pirated, copyright-infringing copy. Please send me an email as I am building a case file against the people doing this. Go to to see the full site and find my contact details. And now, back to the meat of the subject....

Like the site? The page you're reading is free, but if you like what you see and feel you've learned something, a small donation to help pay down my car loan would be appreciated. Thank you.

Engine oil / Motor oil Shelf Life.

It was unclear whether to put this in the FAQ or the main page, so it's in both, because it is a question that gets asked a lot. Typically, the question is along the lines of "GenericAutoSuperStore are having a sale on WickedlySlippy Brand synthetic oil. If I buy it now, how long can I keep if before I use it?"

In general, liquid lubricants (ie. oils, not greases) will remain intact for a number of years. The main factor affecting the life of the oil is the storage condition for the products. Exposure to extreme temperature changes, and moisture will reduce the shelf life of the lubricants. (an increase of 10°C doubles oxidation which halves the shelf life) ie. don't leave it in the sun with the lid off. Best to keep them sealed and unopened.

Technically, engine oils have shelf lives of four to five years. However, as years pass, unused engine oils can become obsolete and fail to meet the technical requirements of current engines. The specs get updated regularly based on new scientific testing procedures and engine requirements. But this is only really a concern for owners of brand new cars who still have engine oil bought for the previous car. An oil that is a number of years old might not be formulated to meet the requirements set for the newer engine.

If an unopened container of engine oil is more than three years old, read the labels to make sure they meet the latest industry standards. If they do meet the current standards, it's wise to take the extra precaution of obtaining oil analysis before using them. An oil analysis will check for key properties of the oil and ensure that it still meets the original manufacturing specs. Of course the cost of getting an analysis done on old oil is probably going to outweigh going and buying fresh stuff. So it's a double-edged sword.
As a general rule, the simpler the oil formulation, the longer the shelf life. The following is a guideline under protected conditions - indoors at about 20°C:

ProductShelf Life
Base Oils, Process Oils3 years
Hydraulic Oils, Compressor Oils, General Purpose Lubricating Oils2 years
Engine Oils and Transmission Oils3 years
Industrial and Automotive Gear Oils2 years
Metal Working and Cutting Oils1 year

The following are signs of storage instability in a lubricant:

Water contamination in a lubricant can be detected by a "milky" appearance of the product.

Next page

Engine oil basics/reading the W numbers/Sludge/Shelf life   High mileage oils/oil prepressurisation/viscosity/oil changes