At the outbreak of the war, Germany’s stockpiles of fuel consisted of a total of 15 million barrels. The campaigns in Norway, Holland, Belgium, and France added another 5 million barrels in booty, and imports from the Soviet Union accounted for 4 million barrels in 1940 and 1.6 million barrels in the first half of 1941. Yet a High Command study in May of 1941 noted that with monthly military requirements for 7.25 million barrels and imports and home production of only 5.35 million barrels, German stocks would be exhausted by August 1941.

http://www.airpower.maxwell.af.mil/a...aug/becker.htm

From 2.8 million barrels in 1938, Romania’s exports to Germany increased to 13 million barrels by 1941, a level that was essentially maintained through 1942 and 1943. Romanian deliveries amounted to 7 million barrels in the first half of 1944 and were not halted until additional raids on Ploesti had been flown in the late spring and summer of 1944.

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