Castor oil
Control line models tend to run a varying mix of fuel however 10% nitromethane, 20% castor oil and 70% methanol is common. Castor oil is sometimes replaced by synthetics, however as control line aircraft typically run at high throttle settings for the entire flight, castor oil generally provides better lubrication and cooling and is thus considered safer for the engine. It is however somewhat viscous and the resulting oil drag can rob some power compared to synthetic oil, and can also lead to "varnishing" of the cylinder. Some older-technology engines commonly used for control line can be very quickly damaged with typical R/C fuels because of low oil content.