Well it specs SAE 90 oil but i'd have to order online, i guess it has to be done though.
SAE 90, but SAE 90 what?
? GL-3? Gl-4? Unless it specifically says that GL-5 is OK, I would NOT USE almost any modern gear lube out there. They're virtually all GL-5 approved, and even if they say "meets or exceeds GL-3 and GL-4", the fact that they also meet GL-5 makes them deadly to many older manual transmission synchronizers.
Redline MT-90 is as good as you're going to get. GL-4, specifically designed to be kind to synchronizers.
Slightly less optimum but still recommended for a lot of manual transmissions that need a 90-weight (for the gears) and GL-3 (for the synchronizers) is an Xw40 engine oil, believe it or not. Same viscosity range as an 80w90 gear oil, but without the synchro-killing sulfurized EP additives that GL-4 and GL-5 90-weights have.