However, by July 1918 there was a shortage in Germany of castor oil,[2] a plant-derived lubricant that the rotaries required, as it could not be easily dissolved into the fuel and because it possessed lubrication qualities superior to mineral oils of the day.[5] A new Voltol-based lubricant, derived from mineral oil, was substituted and was blamed for engine failures on German fighters such as the Fokker E.V, which used the Oberursel Ur.II. It has been suggested that without proper lubricants, the Le Rhône rotary would have been equally failure-prone.[1]