Benefits of Castor Oil


Jul 29, 2000
South America
Here’s some info about castor oil from Bob Durr, the most experienced lubricant scientist in the labs at Conoco. Bob worked on many product development projects.
“You really have to go back to the basics of lubrication to get a better handle on what happens in an engine. 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. Castor oil meets these rather simple requirements in an engine, with only one really severe drawback in that it is thermally unstable.

This unusual instability is the thing that lets castor oil lubricate at temperatures well beyond those at which most synthetics will work. Castor oil has excellent storage stability at room temperatures, but it polymerizes rapidly as the temperature goes up. As it polymerizes, it forms ever-heavier "oils" that are rich in esters. These esters do not even begin to decompose until the temperature hits about 650 degrees F (343 deg C). Castor oil forms huge molecular structures at these elevated temperatures - in other words, as the temperature goes up, the castor oil exposed to these temperatures responds by becoming an even better lubricant! Unfortunately, the end byproduct 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 polyalkylene glycol like Union Carbide's UCON, or their MA 731. This mixture has 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 make it too stable it would no longer offer the unusual high temperature protection that it did before. Castor oil has other unique properties. It is highly polar and has a great affinity for metal surfaces. It has a flash point of only 445 degrees F (229 deg C), but its fire point is about 840 degrees F (449 deg C)! This is very unusual behavior if you consider that polyalkylene glycols flash at about 350-400 degrees F (176-204 deg C)and have a fire point of only about 550 degrees F (288 deg C), or slightly higher. Nearly all of the common synthetics that we use burn off in the combustion chamber if you get 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 walls at temperatures slightly above their flash point.

The same activity can take place in the wrist pin area, depending on engine design. Synthetics also have another interesting feature - they would like to return to the materials from which they were made, 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 the engine too lean and it gets too hot. The synthetic burns or simply vaporizes, but castor oil decomposes into a soft varnish and a series of ester groups that still have powerful lubricity. Good reason for a mix of the two lubricants! ( " 927 " is a mix as described here!) In spite of all this, the synthetics are still excellent lubricants if you know their limitations and work within those limits. Used properly, engine life will be good with either product. Cooked on a lean run, castor oil will win every time. A mix of the two can give the best of both worlds. Like most things in this old life, lubricants are always a compromise of good and bad properties. Synthetics yield a clean engine, while castor oil yields a dirty engine, but at least now you know why!

Maxima CASTOR 927 has under 50% Group 5 ester (true synthetic oil), an unknown % of castor oil, and 10-20% additives to reduce carbon and gum formation.
DATA: 15.2 viscosity @100ºC, 155 viscosity @ 40ºC, viscosity index 98, density .92, 218ºC flash point

From my experience it does create carbon buildup gradually but I've never had a ring sticking problem from using it. But using it my rings last longer than with Motul 800 or any other oil I have tried. And the smell brings back memories of 70's motocross races!

Klotz BeNOL Racing Castor is another good castor oil mix worth trying. Its specs: viscosity 15.7 @ 100C, 166 @ 40C, viscosity index 96, .93 density (estimated), 291C flash point

Here’s the fuel/oil ratios I would recommend for both based on their viscosity at the estimated engine temperature: For a water cooled engine reving to 11,000 RPM use 48:1, and for a water cooled engine reving to 9000 RPM use 54:1. For air cooled engines those ratios drop to 40:1 and 33:1 due to the higher engine temps which lowers oil viscosity (which is the principal way in which oil offers protection).