Castor oil
Normally known by the term "rotary engine", the usually air-cooled radial configuration, rotating-crankcase Otto cycle engines were used by many "pioneer era" (1903-1914) aircraft and World War I combat aircraft. These engines were designed to have a total-loss lubrication system, with the motor oil held in a separate tank from the fuel in the vehicle, and "not" pre-mixed with it as with two-cycle engines, but mixed within the engine instead while running. Castor oil was often used because it lubricates well at the high temperatures found in air-cooled engines, and its tendency to gum is of little consequence in a total loss system, since it is constantly being replenished. Unburned castor oil in the exhaust was a cause of digestive complaints in many WWI airmen.