Interesting analogy that. Not entirely sure BMC/Leyland's fluid recommendations from the 60's/70's were formulated from an engineering standpoint, more to cover up the symptoms of design faults!

The LT95 gearbox in the Range Rover that you refer to, only ran 20w50 engine oil on Leyland's recommendation because it's internal oil pump had a fibre drive, even though a proper gear oil would have been the preferred choice for the gears themselves. Leyland quickly realised after initial trials that the fibre pump gears shat themselves if EP90 was used.

As for the mini, well BMC had no choice but to run engine oil in the gearbox because the engine shares the same oil; one of Issigonis' demands. Packaging won over engineering. That's why almost all but the lowest mileage transverse A-Series engines will wine, jump out of gear and lose synchromesh on second at some time in their life.

A good deal of transverse A-Series engine builders and folk who race minis ironically specify... motorcycle oil! My brother races minis and always uses Silkolene Comp 4 15w50 bike oil, or Millers Trans-M, which is a dedicated car oil for minis containing the same additives as motorcycle oils have.

As I alluded to earlier, I don't think using car oil once or twice, or every now and again in a motorcycle is going to cause impending doom. Using dedicated bike oil is probably the better of the two options (if you can afford it, have the choice and can get hold of it). It is, as has been mentioned several times in this very thread, your choice...

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