hobbsy, it's Byron Fuel that is supposedly good for planes, boats, and heli. Does not specify syn-castor blend ration. Just states that it contains 20% total oil. Runs great in my .46 2-stroke, and seems to run in the OS .91 4-stroke just fine, although I have only run one tank through it on a test stand to date. If anyone has thoughts on fuel brands or such, it would be appreciated. Thanks