So, I know that questions about oil grade, type, etc. have been asked ad nauseum on TT, but I have a question about what my 2006 WR450 manual says about oil weights. It specifically says that in the USA (and Canada) I should only use 20W-40 or 10W-30 depending on temperature. However, it lists numerous weights for non-US bikes. Why is there this difference in what oil is supposed to be used in non-US vs. US bikes?