Chris already has an extensive explanation of oil additives in the oil bible (it's how I found the site), and a separate section on specific additives, but I thought the following link gives a great illustration of why oil additives are basically always bad. Mostly b/c it's got pictures.
http://www.bobistheoilguy.com/images/lucas/lucas.htmThe short of it: Lucas oil additive is tested. This stands as the sole candidate for a 'legitimate' oil additive. It's used extensively by truckers, apparently, and you've probably seen those nifty little oiled gear displays at like every other auto store. It thickens the oil to make it stick to gears, ostensibly providing more sustained lubrication. Turns out, it does do that. The flipside is that any motor oil you use has a fine balance of chemicals to stop it from foaming and doing other nasty things when the engine runs. Introducing an additive upsets this balance, and in the case of Lucas here, the oil turns to foam at high rpms, and fails to lubricate at all.
You can even see this effect for yourself with their own demonstration stands. They have two sets of gears, one with regular oil and the other with some added Lucas. If you spin them both really fast, you'll see that the Lucas side will have a lot more bubbles in it when you're done.
I'm inclined to say this goes for any additive. Anything you add on top of the original formulation will change the chemical makeup to something that neither the oil or additive maker could have predicted or accounted for. And that oil ain't gonna work right.
