While not perfect due to eventual varnish issues, castor based oil is superior. HERE IS WHY: (long read but worth it) For any fluid to act as a lubricant, it must first be "polar" enough to wet the moving surfaces. Next, it must have a high resistance to surface boiling and vaporization at the temperatures encountered. Ideally, the fluid should have "oiliness," which is difficult to measure but generally requires a rather large molecular structure. Even water can be a good lubricant - under the right conditions! Castor oil meets all these simple requirements in an engine, with only one really severe drawback in that it is thermally "unstable." Thermal instability is not really a drawback but a hidden benefit and here's why: This unusual "instability" is the thing that lets castor oil lubricate at temperatures well beyond those at which most synthetics will work! Castor oil is roughly 87% triglyceride ricinoleic acid, which is unique because there is a double bond in the 9th position and a hydroxyl in the 11th position. As the temperature goes up, it loses one molecule of water and becomes a "drying" oil. Castor oil has excellent storage stability at room temperatures, but it polymerizes rapidly as the temperature goes up. As it polymerizes, it forms heavier "oils" rich in esters. These esters don't even begin to decompose until the temperature reaches about 650°. Castor oil forms huge molecular structures at elevated temperatures - in other words, as the temperature goes up, the oil exposed to these temperatures responds by becoming an even better lubricant! Unfortunately, the by-product of this process is what we refer to as "varnish." So, you can't have everything, but you can come close by running a mixture of castor oil with a polyalkylene glycol like UCON. The mix seems to have some synergistic properties, or better properties than either product had alone. As an interesting sidelight, castor oil can be stabilized to a degree by the addition of Vitamin E (Tocopherol) in small quantities, but if you made it too stable, it would no longer offer the unusual high temperature protection that it did before. Castor oil is not normally soluble in ordinary petroleum oils, but if you polymerize it for several hours at 300°F, the polymerized oil becomes soluble. Hydrogenation achieves somewhat the same effect. Castor oil has other unique properties. It is highly polar, or has a great affinity for metal surfaces. It has a flash point of only 445°F, but its fire point is about 840°F! This is very unusual, if you consider that polyalkylene glycols flash at about 350-400°F and have a fire point of only 550°F or slightly higher. Most common synthetics that we now use burn in the combustion chamber if you get off too lean. Castor oil does not, because it is busily forming more and more complex polymers as the temperature goes up. Most synthetics boil on the cylinder at temperatures slightly above their flash point. Synthetics also have another interesting feature - they would like to return to the materials from which they were made. These are usually things like ethylene oxide, complex alcohols, or other less suitable lubricants. This happens very rapidly when a critical temperature is reached. We call this phenomena "unzippering" for obvious reasons. So, you have a choice. Run too lean and the engine gets too hot. The synthetic burns or simply vaporizes. The castor oil decomposes into a soft varnish and a series of ester groups that still have powerful lubricity. So, what oil do you think is best in an aircooled engine that has a tendency to get too hot? If you are wondering what is killing bushing bearings, its running at full throttle where engine is not able to fully dissipate the heat load causing synthetic oils to vaporize and quit working.