Castor oil, lower cost than many candidates. Kinematic viscosity may be an issue.
[size=75]From Wikipedia, the free encyclopedia [/size]
Castor oil is a vegetable oil obtained from the castor bean (technically castor seed as the castor plant, Ricinus communis, is not a member of the bean family). Castor oil (CAS number 8001-79-4) is a colorless to very pale yellow liquid with mild or no odor or taste. Its boiling point is 313 °C (595.4 °F) and its density is 961 kg·m-3. It is a triglyceride in which approximately ninety percent of fatty acid chains are ricinoleic acid. Oleic and linoleic acids are the other significant components.
The structure of the major component of castor oil is shown below:
Ricinoleic acid, a monounsaturated, 18-carbon fatty acid, is unusual in that it has a hydroxyl functional group on the twelfth carbon. This functional group causes ricinoleic acid (and castor oil) to be unusually polar, and also allows chemical derivatization that is not practical with most other seed oils. It is the hydroxyl group which makes castor oil and ricinoleic acid valuable as chemical feedstocks. Compared to other seed oils which lack the hydroxyl group, castor oil demands a higher price. As an example, in July 2007 Indian castor oil sold for about US$0.90 per kilogram (US$0.41 per pound) while US soybean, sunflower and canola oil sold for about US$0.30 per kilogram (US$0.14 per pound)
Castor oil and its derivatives have applications in the manufacturing of soaps, lubricants, hydraulic and brake fluids, paints, dyes, coatings, inks, cold resistant plastics, waxes and polishes, nylon, pharmaceuticals and perfumes.
Sulfonated castor oil, also called Sulfonated (sulfated) castor oil, or Turkey Red Oil, is the only oil that completely disperses in water. It is made by adding sulfuric acid to pure castor oil. This allows easy use for making bath oil products. It was the first synthetic detergent after ordinary Soap. It is used in formulating lubricants, softeners, and dyeing assistants.
The castor seed contains ricin, a toxic protein removed by cold pressing and filtering. However, harvesting castor beans is not without risk,  allergenic compounds found on the plant surface can cause permanent nerve damage, making the harvest of castor beans a human health risk. India, Brazil and China are the major crop producers and the workers suffer harmful side effects from working with these plants. These health issues, in addition to concerns about the toxic byproduct (ricin) from castor oil production, have encouraged the quest for alternative, domestic sources for hydroxy fatty acids. Alternatively, some researchers are trying to genetically modify the castor plant to prevent the synthesis of ricin.