Currently using Red Max 18% synthetic oil & 15% nitro blend, though most of its life (approx 10 yrs) castor/syn blends have been used. Only one head gasket was provided with this motor (I believe it was one of the first batch's distributed to USA before Morris Hobbies picked them up). The motor has been in storage for approx 3 yrs & when I mounted & fired it up, the head gasket blew. i'm fairly confident the head screws were snug because I had just taken the head off for inspection as the motor was slightly gummed up from storage (marvel mystery oil). The bearings felt a little rough but after a tank was run they feel smooth again. I've flown the motor about 5 flights recently without any head gasket. It wants to backfire & sling a prop upon start-up if not using an electric starter, otherwise it both screams, transitions, & idles beautifully. One recent change I did make though was to ditch the OEM tuned muffler & replaced with a Macs Muffler, could this have caused the gasket to blow?