Do they still make T-Bird Randall, maybe in Cal plus I thought the gearboxes drank 30 W non-detergent
I haven't a clue about the Tbird; but if Gallo still makes it, I'll bet they sell it nationwide. Don't recall ever seeing it on a shelf here in CA; but then I don't spend a lot of time in the kind of places that usually sell it.
Triumph did originally recommend motor oil for the gearbox (although there is an early TSB suggesting that gear oil is OK too, without overdrive). But then, about the time the TR3A was (unofficially) introduced, they switched to specifying GL4. Ken Gillanders once wrote that an ex-factory engineer told him that they were having too many countershaft failures, and the GL-4 provided better protection. S-T and later BL continued to specify GL4 through the end of TR6 production, so evidently they felt it worked.
Personally, I really like the MT-90. Doesn't change OD performance much, but makes a noticeable improvement in the main gearbox IMO; in the form of easier, more positive shifts. My synchros are getting somewhat worn now, but with almost 200,000 miles since the main box was overhauled (1997 IIRC), I consider that pretty good service.