Sounds like the main
jet may be gunked up. Beyond half throttle, the other carb
circuits shut down and the main jet does all the heavy lifting, which would explain the condition you described. May even be a little water in the float bowl; it wouldn't hurt to drain the float bowl. If you want to be certain, collect the gas from the float bowl in a clear plastic bottle to see if there's any water. It will be pretty obvious, like salad dressing.
Like skwearpeg said, best to drain the tank and refill with fresh gas. 16 oz. of Seafoam is a lot, most people only use a couple oz. per tank, but for a full system clean, or a "shock treatment" as it's sometimes called, it couldn't hurt. Also, the bike may smoke a little from that much Seafoam, but don't worry, it *will* clean everything out. Don't change the oil until *after* you run the Seafoam.
As for the oil level, make sure you're checking it properly. R*s have a semi-dry sump, so there's a funky procedure for checking the oil level. The bike is supposed to be fully warmed up and you have to stop the bike, do *not* blip the throttle or rev
the motor, let it idle for 15 seconds, then turn off the motor, level the bike, i.e. off the side stand, and check. But even then it can be a bit of a crap shoot to get an accurate reading.