This made engine fumes heavy with smoke from partially burnt oil. Castor oil was the lubricant of choice, as its lubrication properties were unaffected by the presence of the fuel, and its gum-forming tendency was irrelevant in a total-loss lubrication system. An unfortunate side-effect was that World War I pilots inhaled and swallowed a considerable amount of the oil during flight, leading to persistent diarrhoea.[12] Flying clothing worn by rotary engine pilots was routinely soaked with oil.