GL-4 only for the gearbox. Yes it can be hard to find.

GL-5 that claims to be compatible with brass or yellow metals will still cause excessive wear to the synchros. They claim compatibility because it's not corrosive to those metals so can be used with brass components. That is true in itself, but if those brass components are synchros they will still wear excessively due to the way the EP GL-5 oil works. Don't use any GL-5 in the gearbox. GL-5 is designed for use with steel gears only.

Use GL-5 for the diff.