i did some research on this recently and this is generally wat i found.
engine oil commonly used seems to be a 10/40 semi synthetic or 20/50 millers oil
gearbox GL4 only gear oil
rear diff GL5 gear oil.
all seems to depend on your cars mod and wear state.
im using 15/40 semi synthetic engine oil
in my gearbox im using 75/90 ep gear oil that says on the bottle suitable for GL4 applications
and i havent got around to doing the diff yet but i will probably use the same as the gearbox oil unless some one recomends something better.