Oil additives are used to improve the base oil (or oil "base stock") into a better performing lubricant. By utilizing the same base stock, many different oils can be manufactured, each with its own unique properties.

Nearly all motor oils currently being sold have an additive package, whether they are synthetic or petroleum based. Essentially, only API Service SA motor oils have no additives whatsoever, and they are therefore incapable of protecting modern engines. ^[1] Different additives are used depending on the application, e. g. the oil for a direct-injected diesel engine 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).