Here's what Randy Smith - who custom builds the custom AeroProducts PA line of engines used by numerous national and world champion F2B competitors has to say about oil content:

"For motors with bushings and bushed rods like the OS FP , Magnum GP series, Tower, and Brodak’s a 22-25% half-and-half oil mixture works the best. For S.T. .46 51, and .60s and most all ball bearing Stunt motors, a 23% half blend works best. Again the Synthetic blend will help keep the engine clean, and insure long life. If you use all castor in these types, it can stick the ring in the groove , resulting in poor compression and shortened engine life. If you have a ringed engine that castor has gummed up badly, most times running the synthetic blend will free the stuck ring, and the engine will return compression and power for you.

The tuned pipe motors like a little more synthetic and I recommend a 15% synthetic, 7% castor blend or a 20% half and half with 1 ounce of Aero-1 fuel supplement. Although many use 1\2 – 1\2 with great success. This works very well in the Precision Aero , OPS and Max VF engines, Super Tigre Thunder Tiger, AERO TIGER and most all of these type engines..

Four Strokes engines also like the blend, I have found that a 15% synthetic – 3% castor blend works well for them, normal oil percentage is 18 to 20 %. This will vary some from engine to engine, but is a good starting point. Most like 10 to 20% nitro, going up to 25% to 30% in the hot summer weather. Aero-1 Fuel additive can help 4 strokes tremendously, as these engines are lubricated mostly by “blow by” and can run very hot. Fuel and tanks are also very critical for 4 stroke operation. Make sure you have a tank that delivers fuel easily to the engine, as four strokes don’t seem to like having to pull fuel from the tank. Use as short a fuel tank as possible and keep it close to the engine. A lot of people use muffler pressure or pumps to help feed the 4 stroke engines. I have used OS VF pumps, Perry vibration pumps and Perry pressure pumps with my test on 4 strokes. I would suggest,as we do with 2 strokes, to use a Sullivan “Crap trap” fuel filter. They hold a lot of junk ,and have a very good double cone design, that pushes the debris away to the sides and almost never stop up. If you get a stopped up filter on a four stroke ( or 2 stroke for that matter) you can burn the engine up in one flight.

For you guys who absolutely gotta buy the bargain R/C sport fuel…No amount of persuading will convince you otherwise; you at the very least need to add a healthy dose of castor oil. You can roughly figure 1.3 ounces will raise the oil content one percent (i.e. 13 ounces of oil to make 15% oil fuel into 25% oil fuel). This is not recommended and at the very best will usually be a guess, but it is much better than not adding anything at all, and I know people that do this all the time and get it to work for them .Example using Fire Power Cool 15%, pour off 13 ounces and add 13 ounces of Castor, this will be close to 11% nitro, and 24 to 25 % oil , this would make an OK fuel for plain bearing FP type motors, add a little more oil, and you have a fuel that you could run in your Foxes. The final thing I would like to say is to make sure you use an after-run oil between sessions and when you store the motors."