High oil content can't cause overleaning because the engine only sees how much nitro/methanol is being fed into it and that's the only think the needle valve adjusts for. The oil is only along for the ride. If you change oil content then you have to retune to account for the change but the end result is still to have the same amount of nitro/methanol going into the engine. All that 20% oil is likely to do is make the engine last longer.