Why could an army not simply take a navy's fuel, and why did naval fuel grade matter so much?
Short answerHeavy bunker oil and selected steam coals demanded specialized machinery and supply arrangements that made them poor substitutes for another service's fuel.
NotesMost Kriegsmarine surface ships burned heavy bunker fuel that the German Army could not simply divert into its own vehicles. The fuel could require preheating before it would flow and burn effectively, making heating equipment part of the ship's propulsion system. When the Iowa class later changed to a lighter fuel, removing that equipment recovered both weight and operating effort. Coal-fired fleets were similarly sensitive to grade because different coals produced different heat, smoke, and handling results. A later correction in the discussion distinguishes the Royal Navy's preferred bituminous Welsh steam coal from separate anthracite experiments, reinforcing the point that precise fuel properties mattered.