You can use a modern GL5 gear lube in the rear axle. Something like 85w-120 (there are other options). The transmission is a different story. Because of the "yellow" metal parts used in transmissions (such as synchronizers) you should not use most GL5 lubes. A GL4 lube would be better. Some GL5 gear lubes claim yellow metal compatibility, but I would stick with the GL4.
If you search the Barn threads there are endless threads on oil and gear lube! And you will find just about as many recommendations as there are people posting!!!!! It is just about the same as asking what is the best color to paint my car.
