Of course YOU do do NOT mix oil and fuel! Unlike as with primitive 2-stroke-engines in case of the RD250 you put only normal fuel into the fuel tank and inside the oil tank you put only oil! The bike mixes fuel and oil by itself - and uses only very little oil, automatically regulated to an optimum and minimum depending on motor speed and load (my RD 250 used only a little more than 1 liter of oil for about 1000 kilometers).