You got my curiosity up, but I really couldn't find anything. I called Napa here and they do have 20w40 on hand. I guess it gets used in motorcycles that have one reservoir for the motor and the tranny. I read the same thing about using 15w40 in motorcycles also. So, I know that both are used in motorcycles, both are motor oils, and both are fine for wet disk transmissions. However, I haven't found anything saying that 15w40 can be used directly in the transmission. I personally don't think it will hurt anything, because the book is already calling for motor oil being used. To be on the safe side, ya might want to use 20w40 API. I hope you find a better answer!
