You only need the 15w-50 oil if you ride in hot conditions. I Use it in my XR650L during the summer here. It is not uncommon to see temps around 120F in the shade (dont wanna see out in the open. If you use to heavy an oil in the cooler temps it will just creat more friction and make the bike run hotter. My 650 is air cooled and temp around here is something I have to worrie about.