Have a read of this https://www.millersoils.fr/en/the-various-different-types-of-gear-oil-confuse-me-what-does-gl1-gl4-and-gl5-etc-mean/ from Miller Oils which may help clarify the position.
Remember that the Traction Gearbox has synchromesh on 2 of the forward gears AND the oil is shared with the differential so it gets quite complex in choosing the correct oil but all sources I have found are quite clear that anything with GL5 specification should be avoided in classic cars.
David Faulkner