There is a lot of misinformation in John's reasoning for recommending non-detergent 30W oil.
There is nothing wrong with using 90W gear oil in an OD or non OD transmission. 90W gear oil has a very imlar viscosity compared to 30W motor oil. Gear oil and motor oil are rated differently and hence the reason for the different viscosity valves. John’s statement “…90 weight oil was so heavy…” just goes to prove that he does not understand the differences between motor oil and gear oil.
Detergents and bubbles are separate issues. Detergents are used to suspend soot within the oil. Detergent is different than soap, it won’t create bubbles. In an engine, if the oil level is too high, the crank could be partially submersed in oil and as it turns can entrain air into the oil, which will reduce the lubricating properties of the oil, hence the reason to not overfill the sump and the reason anti-foaming agents are added to motor. There is no mechanism within a gearbox to entrain air in the oil.
In terms of what type of oil SHOULD be used in an OD gearbox, why not look at the oil(s) recommended by the auto company that used more ODs than any other: Volvo. Volvo, who put over 400,000 OD units in the their cars specified everything from motor oil to gear oil and finally ATF, so obviously the OD unit is not particular.
Personally I use 75W90 synthetic GL5 gear oil in my Spitfire's J-type overdrive and my MGB's LH-type overdrive.