IMO, there's no way any oil would stay in an open driveline u-joint spinning 2500rpm+. Big truck hub oil might come close to staying in. I've never used or seen used anything other than chassis grease-I've worked at service stations, big truck garages, stealerships, independent repair shops, now a parts store.
Maybe they spec'd oil so the joints would simply wear out sooner, kinda like the Dex-Cool antifreeze specs...planned obsolescence
I use JT6 Mystic in everything.
