I like to run 20% in mine. Yeah grass is hard on it because of the small tires. I really like mine. One thing I did was to cut out the windshield and around the motor. The only couple of problems I had was my own doing. I melted the slipper clutch first. Then a dog ran in front of it and busted the turn buckle and a arm. Finally I was running WOT and hit a bump and turned wrong. Did about 10 somersaults and landed on the front wheel. I busted the tie rod end. Other than that it took me a while to get the motor tuned in, but since then it has run really well. I can run a whole tank with no overheating and keep going. A couple of things you might want to try is cut the grass really short, get some ball bearings for the wheels. And if it keeps overheating cut some air holes in the body. The fuel you are using is fine. It is very oily and will give you good protection. Also try to richen it up a bit. It really does not matter how far out you go as long as its not bogging the engine down.