The Cappo gearbox takes a GL-4 75w90. Castrol products seem to suit it and work well. Both of the following are widely used.
Possibly not. If it's like the Cappos diff then it need GL-5 spec oil which is why a lot of people use the Castrol Universal as it meets both GL-4 and GL-5 specifications. It also means you can fill both at the same time with less than 2 litres.
look at this one
Designed for transmissions and transaxles - helps to slow synchros for easier shifting across a broad temperature range
Contains extreme pressure additives like our 75W90 GL-5 oil, but lacks friction modifiers to balance slipperiness
Improved copper corrosion protection to prolong synchro life
Helps with lock-up on weak limited-slip differentials-compatible with Red Line Limited Slip Friction Modifier for tuning slippage
Also used with clutch-type LSDs in racing for maximum lock up
Recommended for API GL-5, GL-6, MT-1, MIL-L-2105E and SAE J2360