As several other posters have mentioned, Marvel Mystery Oil has worked well for me for years. I bought an Apache years back with a couple of high time O-320's, that would pop and snort on a cold start until warmed up. I added a quart of MMO to each engine, and the problem disappeared almost immediately. This stuff is an excellent upper cylinder lubricant for valves and rings. I plan on adding a pint to my oil changes once my new YIO-360-M1B is broken in. They say the "Mystery" additive is banana oil, whatever, I am a believer in this stuff and the Old Timers swear by it.
RV6 completed 1991 sold.
RV7 #72018 N767T first flight 11/21/2017 150+ hrs.
IO-360M1B MT 3 blade, Dual AFS 5600.
1941 J3 Cub
2018 dues paid.