This is a copy from my truck board.
I don't have the full technical info, but basically Mobil1 had changed the additive package in their Synthetic oil, and decreased the amount of zinc but increased the amount of moly? to compensate. Zinc has great lubrication and consistency, while the moly-whatever is slippery, but has been known to cause ring seating problems...basically zinc=good, moly=poop. This change happened around the end of 06. I was told to look for "SM" on the rating circle on the bottle, and if it has "SM", it's the new junk. I'm gonna try and search google when I get a chance and see what I can find. I got the info from a reputable source, so I may be reconsidering my oil of choice
PerformanceTrucks.net Forums - Just heard some bad news about Mobil1
Has anyone heard of this or have any thoughts??