Join Date: Dec 2014
Location: Perigueux, 24, France
Between about 1935 and 2000 there were millions of cars which used the same oil for the gearbox and the engine with no problems. These were used in many Austin, Morris and British Leyland cars including all minis of that era including the coopers.
Even now there are loads of cars which specify engine oil for use in the gearbox.
There is no such thing as a bad oil as there are only a few major companies who make the stuff, that is why they all comply with international standards. There is such a thing as an oil that is too thick which will take many revolutions of your engine before it has circulated fully on start up.