API rates the engine oil service. SA, SB, SC, SD, SE, SF, SG, SH, SJ, SL, SM, and SN are API rated engine oils.
API Rating Defined. The first letter “S” stands for Spark ignition (gasoline) engines (also referred to as Service categories). The later the second letter is in the alphabet, the newer and more advanced the oil. For example, SC oil was used in vehicles in the 1960s and is not suitable for current vehicles. Sometimes you’ll see designations such as CJ-4. The “C” stands for Compression ignition (diesel) engines (also referred to as Commercial categories).
API Standards. Newer engines are requiring more strenuous standards. When the API determines that current oils cannot meet newer engine needs or when current technology can improve engine oil qualities, it establishes a new set of standards. The key is that SN oils replace all the previous oils and can be used in any spark ignition engine with any of the previous letter designations. Look for the API symbols when identifying the correct engine oil for your vehicle. Over time, API will come out with more advanced oil standards to replace SN oils.