Marvel Mystery Oil is rich in solvents, too, but it is also pretty slippery. Recent (relatively recent -- I've been fiddling with cars for almost 50 years) Ford clutch packs come with instructions that the discs should all be soaked in Ford LSD additive overnite before they are stacked together and assembled (so that all surfaces are fully exposed to the stuff). Perhaps (I'm just guessing, you understand!) it would be good to try to give yours a slow bathing in the stuff somehow, if you still have the rear cover off.