wow, 20:1!!! :D
you're going to foul plugs like I eat pizza man, since the mileage isn't so high why not try a range?
35:1, 40:1, 45:1 ? watch the kind of oil you buy though because some brands will tell you specifically which ratio to mix the fuel in. the Stihl brand of oil i use strongly recommends a 50:1 ratio, it may vary.