The API ( American Petroleum Institute ) provides certifications for engine oil.
Such certifications range from SA for 1930's era vehicles up to the current standard SN. The above linked document from the API lists all grades prior to SJ ( introduced in 1997 ) as obsolete.
For SN it lists:
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.
I see SJ grade oils still on sale at somewhat cheaper prices, and was wondering if there is any real benefit in using the newer SN oils in a vehicle that was originally designed with the SJ grade oils in mind?
I ask about benefit since it seems to be implied by the above document that the newer grade is reverse compatible with recent, older engines - say anything built in the last 20 years.