We recommend only a synthetic /castor oil content for 4-cycle engine applications. This recommendation is currently being relaxed by many 4-cycle engine manufacturers. While some manufacturers blend 4 cycle fuel with a different total oil content than their regular 2 cycle fuels, we have always maintained the exact same oil content for our 4- and 2-cycle fuels. In some cases you may find a manufacturer who will stress that 4-cycle fuel should not be run in his 4-cycle engine. Check with the distributor or dealer on his warranty policy first and then you decide which fuel to run in your 4-cycle engine. You can count on Byron Originals to state the exact oil content on the label so you can determine whether our 4-cycle fuel (at 16% total oil content) is right for your application or if you should move to our Premium 18, (with 18% oil) or our Traditional (with 20% oil). Regardless of your decision, we recommend you limit your nitromethane to either 10% (larger 4 strokes) or 15% (smaller 4 strokes).