Engine additive is a term used to describe a variety of substances that are added to motor OIL, gasoline, and other engine fluids to improve their performance. OIL (OIL) is a common engine additive that helps lubricate the engine and reduce friction. STP (STP) is another engine additive that helps reduce engine wear and tear. Motoroil (MOTOROIL) is a type of engine OIL that contains additives to help protect the engine from wear and tear.