Just to avoid a lengthy oil conversation, 15W40 is more common...well everywhere but in the US where 0,5,10,20w30,40&50 is more common for gas engines. Yes you can get 15W40 here but it's more often used in different applications then in power sports. As long as it has the correct ratings (mostly JASO MA2) you can use anything from 0-15w40. Not over the 40 though as it effects the pump and pressure regulator/relief valve. I always recommend using an oil specifically formulated for use in the application...in this case gas engines and more specifically, high RPM, high heat motorcycle engine oil. Better to have too much protection then not enough. My father always said "Never go cheap on fluids or filters" because rebuilding isn't cheap and you never want it to happen because you saved a buck on oil or a filter. In that spirit.. I use what I consider the best..and one of the more expensive oils out there at over 10 dollars a quart. Full synthetic Castrol Power1 or Power RS 10w40 4T motorcycle racing oil.