To raise the percent oil from 18% to 20%, add 3.2 oz of oil, not 2.56 oz, to a gallon of fuel. 3.2 oz is about 95 ml. What you will end up with is 131.2 oz of fuel that has 20% oil and 9.76% nitro. If you want to end up with fuel that has 20% oil and 10% nitro, you will have to add 3.29 oz of oil and .37 oz of nitro. If you search this forum, you can find links to websites that have calculators that will do this calculation for you.