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Two of the most common viscosities used in modern car engines are 10w30 and 10w40. While the first part of the numbering suggests that these two motor oils are essentially the same, they differ in the second component. Technically, if one really wants to understand the differences between these two engine oil viscosities it is critical to gain an understanding of how the oil grading system works.

The Viscosity of Oil

Before we start deciphering the numbers on these oil viscosity ratings, let us first establish a clear understanding about the viscosity of the oil.

When compared to water, oil is thicker. It is because of this relatively thicker nature that oil is more resistant to flow. For example, if we had a bottle of water and a bottle of oil and we’re going to pour the contents of these bottles at an incline, we can expect water to flow more or at a much faster rate. This is because it is less viscous or less thick than oil. Oil will still flow, however, because it is thicker or more viscous, it is trying to resist the natural pull of gravity.

Viscosity is, thus, an inherent property of all liquids and this is characterized by the magnitude or level of internal friction occurring within the liquid itself. We can, thus, say that there is less internal friction in water such that it is less viscous than oil.

The viscosity of fluids is affected by a number of factors, but most especially temperature. We all know that a bottle of oil left in the refrigerator can turn into a solid block of wax after some time. In like manner, really thick oil can be made thin or less viscous if subjected to high temperatures.

The viscosity of any given oil is inversely proportional to temperature. If the temperature is increased, the viscosity of the oil is decreased, making the oil more fluid like water. Likewise, if the temperature is decreased, the viscosity of the oil is increased, making the oil move sluggishly or very slowly.

Since engine oils are needed to lubricate the different parts of the engine, they should also be able to withstand the temperature fluctuations in the engine. This means that if the engine is cold, the engine oil should not increase its viscosity too much that it will no longer be able to move efficiently throughout the engine (remember, cold temperature means more viscous or thicker oil). This can result in excessive oil temperatures while also increasing friction.

On the other hand, once the engine is already running and generating tremendous heat, the engine oil should also not become too thin or less viscous than it should be that it is already similar to water in viscosity. If this happens, the engine oil will not be able to protect the critical engine parts from overheating anymore since the oil is very thin. This means the different moving parts of the engine will be rubbing against each other without any form of protection.

This is why the viscosity index of an engine oil is a very important parameter for protecting your engine. Generally speaking, the higher the viscosity index, the more stable is its viscosity amid temperature changes.

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Deciphering the Numbers

Going back to our main discussion trying to differentiate an engine oil with a viscosity rating of 10W30 and one with 10W40, you will notice that there are two sets of numbers separated by the letter ‘w’. Let us decipher these two.

We know that viscosity relates to a fluid’s tendency to resist flow. In other words, it is almost similar to the thickness of the liquid. In our case, it’s the ‘thickness’ of engine oil.

The first number in the viscosity rating, before the letter ‘W’, is the viscosity or thickness of the oil in cold temperatures. This is usually tested at a temperature of zero degrees Fahrenheit. As a general rule, the lower the number of this viscosity rating, the more fluid is the oil when subjected to low temperatures. We mentioned in the preceding section that if the temperature is cold, oil has the tendency to become very viscous or very thick. Engine oil manufacturers have to formulate their oils such that it will not turn into a solid block of wax when subjected 0oF.

To illustrate, if we have two engine oils, one rated at 0W20 while the other is at 10W30, we know that ‘0’ is thinner (less viscous) than ‘10’. This means that if the engine was to be subjected to cold temperatures, the oil with a ‘0W’ rating will still be able to circulate throughout the engine and lubricate its many parts. On the other hand, the oil with a ‘10W’ rating will already be thicker; hence, will move slower. In other words, your car will be able to start immediately if it is using a 0W-rated oil. The 10W oil will still be able to start your car, but it will take some cranking to ‘warm’ up the engine and get the oil through all the nooks and crannies of your engine.

Oh, by the way, the letter ‘W’ stands for Winter which is the cold weather designation for the viscosity rating of the oil.

That’s the first part. The second part refers to the viscosity of the engine oil while the engine is in full operation or is working at 212 degrees Fahrenheit. Again, we have to remember that the viscosity of the oil in high temperatures is directly related to its ability to resist becoming very fluid or very thin. If it gets very hot, you’d want the engine oil to remain thick, not watery. This will help protect the many critical heat-sensitive components of your engine. A rule of thumb is that the higher the value of the viscosity rating, the thicker it is in very hot operating conditions.

Let us go back to our example, the 0W20 versus the 10W30. Suppose we run our engines so that they reach 100 degrees Celsius or 212 degrees Fahrenheit. Based on what we have been discussing so far, we know that the 10W30 engine oil will remain thicker at high temperatures than the engine oil with a 0W20 rating. The latter will become thin – watery – a lot faster than the former.

When we put the two together – the cold temperature viscosity rating and the hot temperature viscosity rating – we now have a clear idea of the protective capabilities of our engine oil. To sum it up, we don’t want our engine oil to become very thick when it’s cold in the same way that we don’t want our engine oil to become watery when it’s very hot.

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Differentiating 10W30 from 10W40

If you have been paying attention to what we have been discussing so far, you can now easily differentiate a 10W30 engine oil from a 10W40 motor oil. Since the cold temperature viscosity rating is the same at ‘10W’, we can safely assume that these two motor oils will be able to resist becoming solid at a temperature of zero degrees Fahrenheit.

Focusing more on the second part of the rating – the ‘30’ versus ’40’ – we know that the engine oil with a 10W40 rating will be able to retain its ‘thickness’ or viscosity a lot longer than the engine oil with a rating of 10W30. In very simple words, the 10W40 oil is thicker than the 10W30 oil.

So, When Should You Use Which?

Ideally, you would want to use a 10W30 engine oil if you live in colder climates. Since the environmental temperature will not add to the heat generated by the engine’s operation, you should still be able to maintain the optimum protective capabilities of the engine oil even though the engine is already at full speed.

However, if you live in warmer or hotter regions, you’d definitely want to grab the 10W40 engine oil. This is especially important during hot summer days when the scorching heat of the sun can really amplify the heat generated by your engine. A 10W30 engine oil will still be able to protect your engine, but it will thin out very fast that it may no longer be able to prevent the metallic moving parts from scraping against each other. As such, you will have a much better chance of protecting your engine if you use a 10W40 in hot weather conditions.

In the past, car owners had this habit of replacing their engine oil with one that has a higher viscosity rating. For instance, they will replace a 10W40 with a 20W50 in an effort to maintain adequate lubrication through the already-degraded oil passages. Today, however, this is no longer necessary because of the significant improvements in oil chemistry, machining, and oil filtration retaining the full integrity of the oil passages.

Your car manufacturer recommended a particular engine oil viscosity rating for a reason. It is always in your best interest to heed this recommendation unless you have a much better understanding of just what level of viscosity your engine truly needs.

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Sources:

  1. The Importance of an Oil’s Viscosity, Machinery Lubrication
  2. What are Oil Grades, Total

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