What viscosity oil does a 1982 Honda XR100 dirt bike use?

i just rebuilt my 1982 Honda xr 100 and just good old fashioned 10w 30 motor oil it does just fine .

You need to run straight 30 oil in your motor for it does not have an oil filter, this may not sound right but straight 30 traps dirt and keeps it at the bottom of the motor. This is an old school trick I am not shure what viscosity the factory oil is but in the long run it will pay off to run straight 30. Good luck