What is the meaning of SM in API oil?
Just assigned in alphabetical order. The newer the service category the higher the letter.
American Petroleum Institute
Gasoline Engine Oil Service Classifications
Category Status Service
SN Current Introduced in October 2010 for 2011 and older vehicles, designed to provide improved high temperature deposit protection for pistons, more stringent sludge control, and seal compatibility. API SN with Resource Conserving matches ILSAC GF-5 by combining API SN performance with improved fuel economy, turbocharger protection, emission control system compatibility, and protection of engines operating on ethanol-containing
fuels up to E85.
SM Current For 2010 and older automotive engines.
SL Current For 2004 and older automotive engines.
SJ Current For 2001 and older automotive engines.
SH Obsolete
SG Obsolete
SF Obsolete
SE Obsolete CAUTION - Not suitable for use in gasoline-powered automobile engines built after 1979.
SD Obsolete CAUTION - Not suitable for use in gasoline-powered automobile engines built after 1971. Use in more modern engines may cause unsatisfactory performance or equipment harm.
SC Obsolete CAUTION - Not suitable for use in gasoline-powered automobile engines built after 1967. Use in more modern engines may cause unsatisfactory performance or equipment harm.
SB Obsolete CAUTION - Not suitable for use in gasoline-powered automobile engines built after 1951. Use in more modern engines may cause unsatisfactory performance or equipment harm.
SA Obsolete CAUTION - Not suitable for use in gasoline-powered automobile engines built after 1930. Use in more modern engines may cause unsatisfactory performance or equipment harm.
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The “S” means the oil is meant for gasoline engines. “M” is a quality designation, introduced in late 2004. The next higher spec is “SN,” then “SN Plus.” https://www.oilspecifications.org/api_eolcs.php Check your Owner’s Manual and see what Kia recommends.