API Performance Class for Motor Oils

API classification uses a two-letter system. The first letter indicates which engine type the oil is suited for: "S" stands for spark-ignition (petrol) engines, while "C" stands for compression-ignition (diesel) engines.

The second letter denotes the performance level, ranging from A to J for petrol engines and from C to F for diesel engines. The further along the alphabet, the higher the performance requirements.