I just put Mobil 1 Synthetic in 15W50, and took it right back out. Running 10 40 my tranny was a smooth as any I had ever ridden after about 2500 km. at 3500km I tried 15w40 and my shift action went to hell. Clutch was properly adjusted and it still had way to much clutch drag ( jumped going into first, just like a honda !) It was noticable right away that the shift springs were now a touch on the light side and took extra concentration to shift, don't get me wrong it was still not the worst shift action probably acceptable a few years ago but none the less not as good as it was. Then there was the power it robbed, I could definitly feel a serious difference in how quickly the engine revved, No more third gear corner exit wheelies and an overall boggy feeling. It also ran 5 - 8 drgrees hotter than it did before, I was seeing 110 degrees Celcius regularly. Dumped it out today and went back to 10w40 and it is right back to being an awsome shifting quick revving bike that I had before. The temperature was also back to where it was before have not seen over 100 degrees. I think the extra effort it takes to pump the heavier oil generated more heat, it also took more of the engines power to turn the oil pump. Plus mabey the thicker oil slowed the spinning components.