I believe it's a matter of additive levels and differing philosophy about formulated lubricants.
Amsoil depends heavily on ZDDP as the anti-wear/EP additive and PAO's and polyol esters as their FM additives.
Redline and Mobil attempts to cover all bases with PAO's, polyol and advanced esters as FM's, and moly and ZDDP as AW/EP's.
Schaeffer's doesn't use polyol esters so it depends on a mix of multifunctional additives such as moly, antimony and ELCA's to provide AW/EP and FM functionality.
I do recall being at an Amsoil meeting with my BIL and hearing Al and other's claim they would never put solids in a formulated engine oil. Of course, they also stated that none of their oils would ever contain petroleum base oils, except for minor carrier oils.
Of course, we know that has changed, too!
[ November 26, 2002, 05:31 PM: Message edited by: MolaKule ]