Great thread. Many thanks to Bemiiten for his photos and contribution. Particularly appreciated are the photos of the bearings with covers removed and the brief discussion on mode of failure.
Bemiiten's bearings show no evidence of coagulated muck from "bad syns". Coagulation and resultant oil starvation is one failure mode alluded to by Getrag/ZWN development engineers in Kari Prager's translation of the coffe chat reprinted on Anton's pages. Regarding additives and presumably certain syns which are not miscible with yak grease, they say:
["We therefore rigorously recommend against their use"].
"Clean bearings" are not common in gear boxes and the refiners of most syn GL5's do not formulate for compatibility with yak grease. Nor is there any labeling convention that aids consumers in selection of a miscible lube.
I postulate that the grease is only there for initial lubrication. It must wash out fairly quickly with miscible hot oil. On the other hand, you'll likely encounter coagulation followed by bearing lubrication starvation if one of the cheap syn GL5's are used, even briefly. Once you have the coagulation I doubt that you can ever be rid of it w/o new bearings.
for sale: 1968 R 69 S-us original condition