Kia Ora
Another kiwi from the the Z-land, good looking 2 you got there, I'll swap you my engine bay for your body paint
I actually just blew my box as well it went in style throwing gear teeth all the way. Kinda looks like the place I brought my mkI from as well.
There are a few threads on gearbox oil here generally any good brand synthetic MT fluid shouldn't give many problems, I used 75-90 Valvoline in my last box that went bang, syncros were good and it shifted smooth. Some people swear by such and such and others something else but it depends on box condition and mileage above most things.