according to the American Petrolium Institute (API) SL, SM & SN are all acceptable and current. SJ is now almost obselete as well as all oils previous to it.
it's always best to use the highest API rating but SN is literally a few months old still so SL & SM are still the industry standards (usually SL for minerals & SM for synthetics).
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