I did a post on Classic Olds but never really found my answer til my last search. My understanding is a ##W40 or a ##W50 weight oil is exempt from the 800 ppm zddp SN maximum which is just weird and doesn't really make sense. It does make me rethink certain oil I assumed were no good because of the SN rating. VR1 20W50 is available at Walmart in SN rating which is supposed to have 1300 ppm zinc and 1400 phosphorus, according to what I have found. To 10W30 is not in store and supposedly SL rated. I have some SL Defy in 5W20 and 5W30 and some SG, SL/SM motorcycle in 10W40 and 5W50. The only downside to motorcycle oil is no friction modifiers.