P-relloR6

I got a 2003 yamaha R6 and my buddy just bought a bike and wanted to go for a quick ride around town. I threw the battery back in it started it up let it warm up for a while and went. It rode great for the first 10 - 15 mins then it start to bog down and sound like a 450 four stroke when I hit the throttle. If I rolled on the throttle very slowly it would run fine but when I would try to clutch it up or gun it it would start boggin down and made that sound. Any clue as to what it could be? . . . . heres a few factors: it was stored in a shed, The air filter was out, and oh another strange thing was when i checked the oil it overflowed out of the dipstick hole?!?! I changed the oil around april 09 and there was never that much in there. i dont know if someone added oil to be a jackass or what