Oil is a highly engineered product, and using oil additives really messes with the careful chemistry of engine oil. If you use a high quality full synthetic oil, it is even more critical not to mess with it. Engine oil has all the additives it needs put in already. Fuel additives are a different issue, since fuel isn't as carefully engineered as engine oil and other lubricants are. If your engine oil isn't engineered well, for god sakes stop using that brand.
