Lyle, what are you driving?... what is the tappet design of the engine?... flat tappets or roller followers?

SL oil has more ZDDP (zinc) than SM oil, which is better for tappet designs, but not necessarily a "must-have" unless the engine is running a hot cam with high spring tensions, such as an altered engine. SM oils have enough ZDDP content for most engine applications.

Though there is a smidgen of truth to what he said, he was throwing his weight around more than anything else. Spend some time on bobistheoilguy.com and you'll get details on this issue.