SM rating is the NEWEST rating of motor oil. and the lowest ZINC (ZDDP) and phosphorus yet.. soon to be phased out completely. in my own personal experiance ZDDP is only needed for breakin with flat tappet cams. another thing people seem to forget is that ZDDP is ONLY there in case of metal to metal contact. now, people are thinking.. well what about startup? theres always a film of oil on your cam lobes. oil is a form of liquid. liquids CANNOT be compressed. so therefore there is no metal to metal contact. if your engine takes ALONG time to register oil pressure after initial startup.. you need a new oil pump to help fight the wear on a camshaft. IF there is an oiling problem, the oil get hot and like any other liquid it "boils" off. for a lack of a better term. when that happens, NO amount of ZDDP will help. like i said before, killer valvesprings and/or oiling problems will kill the camshaft. even with ideal conditions, killer spring rates put tremendous pressure per cycle of the spring.. and the pressure on the oil will transfer to the cam lobe causing it to wear out over time. ie: kinda like a waterjet.. but obviously not as extream or fast acting. thats why those cams/valvesprings need replacing more often. hope this makes some sence to you.