Oil is usually not required to build units but to move units. The Germans in WW2 had 150 or so divisions in the east, but hardly enough fuel for all. Most units were not motorized and walked by foot, using horse waggons for supplies. They often used trains (-> coal) to move armies. The 1942 summer offensive did target sowjet oil fields, not Moscow, while most of the front did not move much. The only nations with an abundance of oil in WW2 were the USA and Great Britain, controlling most of the known and not destroyed global oil reserves. If your neighbours have oil and you don't, conquer it or trade for it.