Never, never run any oil in a Muncie or Super T-10 Borg Warner but the highest grade gear oil you can get 85-140 Pennzoil perferably
The muncie was designed in the early 1960's and the idea is to give protection, a layer of gear oil between the gears. Engineers at that time did testing and discovered heavy weight gear oil gave the best protection.
According to the engineers at General Motors, they only warranted the trans back then if you ran the 90 wt gear oil and according to legand Joe LIberty of Liberty Gears, he told me that he agrees
I have a standard shift Transmission shop. Sometime after the snythethics came out people were asking me about it and I could not give them a truthful answer. So one of the companies that I was doing business with was Joe Liberty Gears. He makes a variety of transmissions for the Nascar circut, drag racing, clutchless transmissions, air shifted transmissions and many others. One day when I was on the phone with him this is what he told me about lubricating the muncie trans. He said that of all the thousands of muncie transmissions that had come to his shop for repair, the bulk of them were broken because they had the wrong fluid in them. Further that only 90 wt gear oil offered protection against breakage and that while a lighter fluid might enable the trans to shift a slight bit easier that it was not worth the risk to steer away from 90 wt gear oil.For some it would be temp variance, I use LE syn. in all my gear boxes. Lubes better and turns more freely.