The different acronyms above specify the certification and standard of your oil. The American Petroleum Institute (API) sets the industry-recognized standards for petroleum and oil products, and the letters following “API” usually note the quality of a certain oil. The highest standard for gas engines is SN, with SM, SL, and SJ following them. All other “S” acronyms, from SA to SH, are currently defined as obsolete and not recommended in modern vehicles. For diesel engines, the top categories are CK-4 or FA-4, depending on your engine.