Newer bikes (same as cars/trucks) have tighter tolerances. Thicker oil doesn't flow into or though those spots so good and you can actually damage a motor by running too thick, especially at cold startup. If it is called for originally then the thicker is fine and MAY ( no way to prove it) be better in those engines, but probably only in hard use situations(aka racing). Newer engines, if 10w-40 is recommended, it is best to stay with it. The viscosity is one big reason why I like to use full syn, much more temp stable and flows similar whether hot or cold(think winter 10F), but regular oil will thicken up significantly in the cold and can starve areas of protection.
I'm starting to stick to 15w-40 for summer and 10w-40 for fall/ spring. In real hot months(80F +) I will top off with 20w-40, but make sure to change when temps cool down. This is in my XT600 and Honda XL 600 , both state 20w for summer in warmer temps.
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