GL5 is so common, and most people are so ignorant of the difference (GL4 only for gearboxes with no yellow metal components!) that most just use GL5 anyway. I think at one time GL4 was quite common and has become less common. Your average mechanic seems to think that GL5 is better and will do both, in the same way you can replace DOT 3 with DOT 4 brake fluid. It is not better it is just different!
With this in mind, I think the reputation T2's gained for 'weak' gearboxes is down to them regularly being abused with the wrong oil! I know a clutch and gearbox specialist that only uses GL5 (and keeps no LSD oil) and is adamant that these differences don't matter and they work on hundreds of cars a week
so my advice to anyone with a 'noisy' T2 box is to change the oil ASAP to slow down the process of damage!