The SL rated oil has a higher amount of ZDDP. They reduced it in the SM because the feds mandated a longer warranty on the cats. ZDDP has an adverse effect on the metals inside the cat, albeit very small.
But not all SL oils meet ILSAC GF-4 for reduced cam/lifter wear.
SM oils do, indicating probably better oil base stocks and advanced additive package to pull this off with lower ZDDP content.
SLs are better for some things but probably not for what you are driving.
