Simply API SA.........SM.. apply to petrol/gasoline engines and CA..........CI4....etc apply to Diesel engines. As engines develope then new requirements from the oil are necessary. Each new requirement is tested through a series of engine and rig tests and is signified by API SG/ CH4 The 4 signifies 4 stroke engine, so the oil is to a specified pereformance for gasoline engines and 4 stroke disesel engines.