Oil additives are chemical compounds that improve the lubricant performance of base oil (or oil "base stock"). The manufacturer of many oils can use the same base stock for each formulation and can choose different additives for each use. Additives comprise up to 5% by weight of some oils.
Nearly all commercial motor oils contain additives, whether the oils are synthetic or petroleum based. Essentially, only the American Petroleum Institute (API) Service SA motor oils have no additives, and they are therefore incapable of protecting modern engines. The choice of additives is determined by the use, e.g. the oil for a diesel engine with direct injection in a pickup truck (API Service CJ-4) has different additives than the oil used in a small gasoline-powered outboard motor on a boat (2-cycle engine oil).
