The US didn't specify one oil for the differentials, transfer cases and transmissions because it was the best practice, they did it because it simplified the supply logistics without compromising function and reliability too much.
The gears, and other parts in the transfer case and transmission don't need the EP additives that are essential for long life in the differentials. The EP additives really add nothing of benefit to the transmission.... some, including the manufacturer of the transmission, claim they make the transmission run less well, compromising the function of the sychronizers. I think EP additives could also be the cause of 2nd gear bearing failures when operated in the 5th gear position...already a known problem (Check out the threads by Tom Bauer aka M-35Tom)
Spicer recommended SAE 50 motor oil in the 3053 transmission. It is exactly the same viscosity as SAE 90 gear oil, but without the EP additives. I have friends who claim that the winter performance of SAE 50 in the 3053 is vastly superior to the performance with SAE 90 gear oil, but I cannot comment on that.
We are in a much better position logistically than the US government was. We can afford to use multiple lubricants just because we get marginally better performance. If you are changing the oils, perhaps it would be a good time to do what the manufacturers of the various parts recommend?
When I next change mine, I am going to use a yellow metal safe GL5 in the differentials... probably NAPA.., and SAE50 motor oil in the transmission and transfer case.
I'm not going to use any of the super dooper Extreme Pressure additive gear oils. And I certainly am not going to add any oil thickeners.
-Chuck
OBTW, this is not a comment on Mystic's quality. I use their GL2 greases on some of my machinery.
