The API SA service classification was followed by SB, SC, SD, SE, SF, SG, SH, SJ, SL, SM and to its current level of API SN.
With regards to "API SM," The American Petroleum Institute (API) Says:
Status: Current
Service: For all automotive engines presently in use. Introduced November 30, 2004. SM oils are designed to provide improved oxidation resistance, improved deposit protection, better wear protection, and better low-temperature performance over the life of the oil. Some SM oils may also meet the latest ILSAC specification and/or qualify as Energy Conserving.
API SG
Advisory: Relatively easy to find on the shelves at C-stores and gas stations. Commonly priced close to that of products meeting current specifications. For 1993 and older engines. Check owner's manual to assure appropriate service category and viscosity grade.
With regards to "API SG," The American Petroleum Institute (API) Says:
Status: "Obsolete"
Service: "For 1993 and older engines"
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