Another mystery (to me):
Do the phospate films have a preference for certain materials? From the stress activated process it would seem that the tribofilm would form on the hardest material (crank journal) and possibly not on the softer one (bearing).
I've read a study about polyol esters where the investigators showed that the esters like iron oxide better than iron itself, but dislike Aluminium oxide and prefer Aluminium. Aluminium oxide is very hard and brittle, so a good candidate for ZDDP film formation?