If I recall, the manual for my 2004 wr450 says 20w 40, but I went to the shop and they didn't have any in stock. The guy told me 10w 50 would work and probably would be better for the hot weather. He said: The 10W means it is not as thick during starting and will actually make it easier to start than using 20w While the 40 is not as thick when warmed up as 50, so the 50 means it will give me better protection in the hot weather. Can someone please confirm? I'm going to do a bit of research and will report back my findings, but I'd like to know what you guys are running and if my understanding is correct. Thanks!