Here is a link to quantum mechanics with a very interesting article on overdrive oil.
https://www.quantumechanics.com/qm-htm/topic1.htm
In a nutshell, John says go with 30w non-detergent.
If you poke around a bit on his site, you can also find where he talks about the thrust washers being a particular weak spot in the TR2-6 gearbox; and if you pay him enough money he will upgrade them to roller bearings. Oddly enough, the thrust washers were also mentioned in the factory's reason why they switched to specifying gear oil.
Triumph sold hundreds of thousands of overdrives over a roughly 15 year period after specifically changing their earlier recommendation to gear oil. If gear oil would make the OD fail instantly, as John said, you'd think someone would have noticed when the units were new.
Another point, the web article does not specify which type of OD he was working on, or what car it came out of. They aren't all the same!
We could also talk about how 90 weight gear oil at room temperature (presumably where John did that test, although he didn't say that either) is actually thinner (less viscous) than cold 30 weight. The test results make no sense, unless cold weather also makes the OD malfunction.