For motorcycle gearboxes with wet clutch you MUST use oil without molybdenum additives. To be safe, buy motorcycle oil from a powersports place. It should have a rating of MA or MB.
For diesels, you need a Cx rated oil, not an Sx rated oil (for gasoline engines.) On a tired one, use a Cx oil of higher viscosity.
In my kart engines, I run the same thing as in my cars, synthetic blend automotive oil (valvoline usually) usually 10-30 winter, 1040 or 2050 summer. I try to change it every 50 hours or so. I don't let it get black looking. For certain, I never let it get as bad as it was when I bought the cars used.
