05 WR450 Motor Oil why 20-40

I have looked, but have not found an answer to this. I just converted from running Hondas, and I always ran Honda 10-40 with very frequent changes. Now Yamaha says to run 20-40, and they are the only ones who seem to make a 20-40. I have a case of 10-40 and a case of 20-50, and I just can not understand why it wont work. The 10 side will actually get flowing faster, and the top is the same weight. Any comments on running 10-40 until the temp gets above 70 then going to the 20-50 in a WR. :thumbsup:

Both of those oils will work fine. I only run 20-40 because i run yamalube in my wr and that is their only nonsynthetic weight they offer and its cheap. Either oil will work fine. I have 10-30,20-50, 5-30, and 10-40 in my quads without a problem.

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