Assuming we're talking about percents by volume , the amount of oil to add to a gallon of 18% oil fuel to raise the oil content to 20% is 3.2 oz . If you want to caluculate it 'by hand', the formula is: Oz of oil to add to a gallon = ((Desired % - Orig. %) * 128)/(100%-Desired %) I built an excel spread sheet to this and several other fuel calculations (% by weight to % by volume, mixing two fuels together, etc.) Anyone who wants it can E-mail me and I'll send it to you.