As someone who formulates MTF for my living - don't use simply a GL-4 or GL-5, that won't be good for your gearbox as it doesn't necessarily have friction modifiers for the synchros. You need a 75w/80 dedicated MTF from a decent brand - gl4 and gl5 are specifications for pretty much wear protection alone - 4 being suitable for an MT and 5 for axles - it says nothing about how the oil will perform in the other things the transmission needs. A lack of friction performance can really impact shift quality, and also, more seriously can wear out your synchros - so the right fluid is very important. Gearboxes are very different to engines and there aren't industry wide specifications - each manufacturer will have the fluid they use tailored to their hardware - so there it is usually best to stick with it unless you have a compelling reason to do otherwise.