I have a 1983 DT125 with 1100 miles on it. I notice from the Haynes manual it needs 10 40 oil in the engine/gearbox. I have synthetic 10/40, will that be ok or does an old bike like this need mineral oil ? Be great if anyone can help with this.-Thanks, Nick

Wet clutch so shake any ideas of using your car oil. Get some proper motorcycle oil, synthetic or otherwise doesent really matter in something like a dt box, its just a basic oil bath in truth. No point spanking money on expensive oils. that gearbox is that strong it could probably run happily in old chip fat.

You don't want to use car oil as that has components to reduce friction which can weaken your clutch.