You are correct in assuming the tighter the clearance the more fuel you have to run. Yes they are meant to run on pump gas but purity an consistancy are a problem plus the lead in the race gas acts as a lube. So by using race gas you know where you are everytime. Myself we use amsoil dominator when that isn`t allowed I use some sort of castor and I use alot compaired to others 8 0z a gallon, does smoke alot after warm up but that keeps the rod happy and on a 20 mm bearing basically the same as a kt100 it is a weak point.
