I'm a VP dealer, and have been selling and using race fuel for 5 years. VP has been putting warning lables on U4 and other oxygenated fuels they sell for about 6 months now. Some users have had problems, others have not, so it's a disclaimer on VPs part. If you use any type of oxygenated race fuel, you should run some pump gas through your system once you are done racing for the weekend. I've never had a gummed system, but I have had some corroding to the inside of my gas tank, when I left MR9 in there for a month after the end of last season.
U4 will give your stock bike a noticable boost just by pouring it in, have your bike jetted, or remapped, for even better results. Big compression, there's VP U2 which is higher octane.
