Put on a helmet. Plenty of threads regarding tranny lube and as many opinions. I have had 80W/90 in it and it worked. I upgraded the overdrive and now run a no frills 40W, as specified by the overdrive vendor. Shifts great.
The military, like any large fleet, likes to standardize to simplify the logistics chain. 80W/90 in the pumpkins, tranny, transfer case and winch drive makes sense for them. Their choice always balances the needs of the machinery with cost and supply - and the machinery does not always win. Back in the day, Spicer seemed to prefer a 30W oil for the tranny.
FWIW, make sure you oil is yellow metal safe. Most gear oils nowadays are. GL-5 by itself is not, but if it is labelled multi-spec, such as GL-5, GL-4, GL-3, it is fine. Some say to stay away from motor oil types or non-detergent oil, because they do not have the high pressure additives good for transmissions and have other additives that transmissions do not need.
I put Lucas stabilizer in my engine and tell myself that I can see the difference in how fast oil pressure rises. Is it true? I want to believe it. I have no experience with it in the tranny or transfer case.