Being that the oil is a grayish color to me means that it is exhaust residue. I would however seal the fuel tank compartment if not already done and attempt to keep the residue from getting in. Try tuning the engine with the airplane upside down. More details such as prop size, fuel brand, nitro content and age of fuel, glow plug make and number. Are you running muffler pressure? The more we know about your setup the better we can help.