My understanding is Synthetic oil is lower in viscosity, has a higher flash point (400 degrees) and it lubricates better.  But it has one big down side when it reaches the flash point synthetic oil completely vaporizes.  Leaving no lubrication for the engine this is where castor comes in, when castor reaches flash point it leaves a layer of residue on the surface of the metal that acts as a lubricant and protects the engine from metal on metal destruction.  My friend Larry Renger also tells me that castor is better at cooling, hence something you want in a Fox engine.  I will let Larry explain how that works.  For modern engines, and especially ABC, AAC, and ANC synthetic should be the majority of the lubricant at least that's how it works in the R/C world.  In the C/L world the addition of some amount of castor, and I really don't know how much is probably good for protection against a lean run where the temperature goes above flash point.

The more "fuel in your fuel", the longer the engine will run on a given amount. If you just can't get in enough laps out of a tank, add some Coleman Stove & Lantern fuel. One ounce per quart will make a noticeable difference.  Steve

 

Goran Olsson has a good write up. The chemistry is explained.

All castor fuel will get more mileage.

   Not necessarily. Either castor or synthetic is (or should be) an inert component. Depending on which synthetic you uses and its viscosity, you may get less oil drag or more oil drag. If you are running at the same output power level, you should be able to get more mileage with a lower-viscosity oil, or less if it is particularly heavy.

    There's a lot more going on with fuel viscosity and oil drag than most people suspect - and not just on the lubrication and heat side. For example, what controls the ability to draw fuel?  How much suction there is and *how hard it is to pull the fuel through the lines*. One of the more important discoveries in recent years is the effect that fuel viscosity has on fuel draw and subsequently the in-flight performance. Just because theres no issue drawing 8 ounces of fuel through a .020 tube *on average* doesn't mean that the transient effects are unimportant. We learned our lesson from 4-strokes, even though niether David nor I ever actually flew one seriously. Just about everything interesting we have learned about engines in the last 10 years has been in the area of fuel delivery.

     Brett