What API SN means
API SN is a service category from the American Petroleum Institute (API) for oils used in gasoline passenger-car and light-truck engines. API categories that begin with “S” cover spark-ignition (gasoline) engines, and each new category generally adds or tightens performance requirements compared with the one before it. API SN was the current gasoline category for a period before being replaced by API SP.
A category like API SN describes what an oil is tested and approved to do — for example how it protects against deposits, wear, and oxidation. This is separate from the viscosity grade, such as 5W-30, which describes how the oil flows at different temperatures. Your engine needs an oil that meets both the specification and the grade listed in your manual.
How API SN relates to API SP
API SP replaced API SN as the newer gasoline service classification, and API SQ followed SP with the ILSAC GF-7 generation. As with earlier transitions, API designs new “S” categories with backward compatibility in mind, so an API SP or API SQ oil in the correct viscosity grade is generally appropriate where a manual originally called for API SN.
There is also a related supplemental designation, API SN PLUS, that was introduced between SN and SP to address certain engine concerns. If your manual specifically lists SN or SN PLUS, a current SP or SQ oil in the right grade usually meets or exceeds those requirements, but OEM approvals and manual exceptions still matter.
What to follow
The most reliable guide is your owner’s manual. It states the API category and the viscosity grade your engine was designed for, and it may also list other approvals. When in doubt, match what the manual specifies rather than substituting based on category names alone, and confirm any substitution against the manual before use.