i was told to use fully synthetic Castrol TAF-X 75W90
(not sure if its still called by the same name as i think some castrol oils have had recent name changes!!!)
think the same oil is used for the gearbox and diffs? but CHECK and confirm with other people before using to make sure!!!

when you remove or replace the gbox, be sure to check pipes for the heater/rad etc as these pipes tend to crack (when the engine moves they get squished and creased) causing coolant to leak from them etc

good luck, all should go well and you'll end up with a nicely serviced gearbox