I do have overdrive, and I did notice a definite difference when switching to MT-90. Not so much with the overdrive, but the main box definitely shifts better with MT-90 in it.
In addition, Triumphs specifically switched away from even allowing non-detergent motor oil to be used, because the bearings in the main gearbox need the extra protection of GL4. If you poke around on John Esposito's web site, you'll find where he also recommends upgrading the bearings on the countershaft, because of this weakness.
John has a very good reputation, but his report on the effects of running GL4 in an overdrive makes no sense to me. Triumph delivered the cars with GL4 in them, and would have been hit with a hideous warranty bill if it had cause the OD to overpressure instantly. Obviously that did not happen. I can only guess that perhaps John was not using an A-type OD as fitted to early Triumphs. (I also run MT-90 in my J-types, where it also works very well.)
Or perhaps he made the same mistake that I did, and shimmed the accumulator spring until it reached coil bind without fully uncovering the relief ports. My test setup turns relatively slowly, so the pressure looked OK, but it went too high when turning 5000+ rpm. Taking out one of the shims solved the problem.