is the oil carried in the crankcase, sump, or oil pan of a reciprocating internal combustion engine to lubricate all major engine parts; also used in reciprocating compressors and in steam engines of crankcase design. In automotive applications, it is the function of the engine oil not only to lubricate, but to cool hot engine parts, keep the engine free of rust and deposits (see engine deposits), and seal the rings and valves against leakage of combustion gases. Additives in the oil greatly enhance its ability to prevent excessive wear and deposit buildup, and they increase its resistance to oxidation and deterioration at high temperatures. Additive content in a single-viscosity-grade oil is typically around 10 mass percent, and in a multi-grade oil, 15-20 percent.

Also see energy conserving, API Engine Service Categories, military specifications for engine oils, SAE engine oil viscosity classification.