What kind of motor oil is used in the 1982 Yamaha 250 XT four stroke dirt bike?

The motor oil to use in a 1982 Yamaha 250 XT 4-stroke dirt bike is a 10-30 or 10-40 weight oil. Castrol GTX Motor Oil that is of this weight is the best oil to use in off road bikes, because it is a cleaner oil. It will not gum up the works of smaller engines like regular motor oil.