Different theories about it discussed have ranged from valvetrain wear (Amsoil rep of course) to a chemical interaction from Fe that is actually contained in the compound of Mobil 1. I'll post the exact comments used from the chemist on the board. Either way, it's annoying and I've since decided to change to another oil in case it is valvetrain wear.
quote:M1 and many oils start off with a certain level of Fe compound in their formulations.
The anti-wear compounds react with the iron and steel to form ferrous-phospates, ferrous-borates, etc. Some of the ferrous (iron) compounds are liberated into oil when separated from the surfacial film