Oil additives are marketing gimmicks that make you think you're taking care of your engine. Avoid them like the plague. I researched several of them a few years ago and am convinced that they do little or no good but can cause great harm to your engine.
The ones that contain PTFEs put Teflon particles in your engine. Sounds good because Teflon is non-stick right? Wrong. Dupont (who owns Teflon) states that Teflon is harmful to internal combustion engines. They have to sell it to oil additive companies because it's a free market, but Dupont won't allow the use of the name "Teflon" because of liability.
Other additives bond slippery compounds to cylinder walls for friction and heat reduction. Sounds good right? Wrong. It takes a corrosive acidic action to chemically bond the additive metal to the cylinder wall. Also, the slippery compounds are softer than the metal they replace. They wear away, leaving pits and corrosion in the engine, unless you replenish them regularly with more corrosive additive.
Another reason to avoid oil additives is their secret "proprietary" formulas. You don't know what's in them, and their formulas change all the time due to legal actions and pre-emptive legal defense. They are not to be trusted.
For the best engine protection, use a synthetic oil GM-spec'ed for your engine (like Mobil1) and change it at recommended intervals.
Here's a couple of links to more info.
http://www.carbibles.com/snakeoil.html
http://skepdic.com/slick50.html