There was a useful article on on oil in VMX, written by an oil specialist. Concerning ATF he said many racers used it but he would rather use something like straight 30 as ATF "could make the clutch woolly". I used to run an SP400 single on car oil. I have a pile of clutch plates 6" high, all unworn and slipping. Heavy clutch springs were a popular mod in the eighties.....
I recently bought a Guzzi Nuovo Falcone. It used to get clutch slip. I changed the oil three times and the clutch stopped slipping. The previous owner had been using car oil. I think ATF uses friction modifiers similar to car oils so I wouldn't use it in a clutch case.

If the clutch chaincase cover was fitted with the steel cap/plug,the gear box filler cap/plug was also steel but was plain with no inscription on it,the same also with the aluminium caps/plugs.
below,steel chaincase and gear box caps/plugs.


below,alloy gearbox cap,there is also a slotted version.


below,unknown cap "USE ENGINE OIL" does fit the Triumph chaincase and gearbox , made from steel but knurled edge,which is not typical of Triumph.