Hi Oily,
Well I would say that you have a flywheel leak, either seal or plugs, there is around 9 pints capacity and it was always reckoned that if you had to put in one pint or more, then it should be rectified. With a low oil level in the flywheel, the oil has to work harder and thus overheats.....this may be why drive was lost until it cooled down. I have actually come across a Saracen that ran low and the flywheel inner member expanded that much it locked into the flwheel, you could see the siezure of alloy in it.
The gearbox oil is SAE 30 engine oil..........although when in service latterly, a multigrade was used, in engine and gearbox, OMD75 then OMD80. All other levels, ie transfer and bevel boxes, hubs, etc are SAE 80w/90 or 90EP. If you gearbox smells burnt, then bands could have been slipping, adjustments should be checked as the self adjusters may well be out of range if worn badly and you can pedal up until the cows come home, with out any adjustment taking place.
Richard