It's a common misconception that adding oil reduces the nitro content. It only does this if you're using the total of oil/methanol/nitro. The engine only runs on (and is tuned for) the flow of methanol/nitro through the needle valve. Adding oil doesn't change the ratios of methanol/nitro in the total fuel mix. Put it this way, if you had a fuel with 10% oil, 45% methanol and 45% nitro then the ratio of methanol to nitro is 1:1 (or 50/50 if you like). No matter how much oil you might add to that mix the ratio of methanol/nitro will never change.