Chili said:
What do you base that deduction on? Most Used Oil Analysis data that I've seen from MX bikes with shared tranny/gearbox oil suggest that almost no oils are still in grade after 2 hours use.
Yes, but does that really matter?
Since we don't operate in extremely cold environments, an oil shearing out of viscosity grade on the cold end is irrelevant for our uses. I don't care how the oil works at 32F since at that temperature my butt is inside where it is warm. If it shears down to a straight 30wt it is still acceptable for hot-weather uses.
With that said, oil is cheap; transmissions and labor are not. It is much better to change it often with cheap oil than to change it infrequently with some motorcycle gearbox wonder oil. The system has no filters in it so the only way to remove contaminants is by changing the fluid. If the cost of oil is too much for you, then you really need to find a cheaper hobby as this one is out of your price range.
Besides, it is an excuse to drink a few beers while you wait for that quart to drain out of the crankcase. At least for those of us of legal age.