General Synthetic gearbox oil

[wilharper2000]

I was wondering whether synthetic 90w gearbox oil can be used in a 4 speed panda. I have got 20w-50 engine oil in mine at the moment and I have found the gearbox is getting quite noisy even though its almost overflowing out of the fill plug.
As far as I can see the 'correct' gearbox oils are
Castrol ST90 (hard to get)
Millers Green Gear Oil 90 (expensive through the website)
or Fiat Zc90 (impossible to find)

Is castrol smx-s or syntrax universal compatible with this gearbox, also what about Mobilube 1 SHC 75W90.

Thanks

Will