not 100% on this. but i think only certain boxes take gl5. td5 for example. something to do with whatever is in gl5 eats bits of your box. gl4 seems to be safe though.
Diffs and transfer box are 80/90 gearbox is mtf or ct33 depending on age, don't worry too much about gl5 oil technology has come a long way since sulphates were introduced for gl5 but if your being passim instil gl5 in the diggs gl4 in the transfer box
Smith & Allan are the people I use for all my oils and fluids for the bike and cars. You'll find their outlet shop on fleabay. I've always found them pretty good and fast deliveries too. They also sell any quantities from 1-200litres.