Oil is needed for general upgrades, Napoleon and MacArthur from scratch and every general from level 10 onwards. Once you reach that level, you'll need 40k+ oil aevery 2 days to not have your armory stay idle. Factory units and air field units upgrades take a lot of oil as well.
I had 135k oil sitting in storage for weeks while my armory was busy with regular troop upgrades using food. If I had to do it again, I would have alternated food and oil upgrades to never have overflowing oil. It's quite rare to find, especially now that people are using it a lot with new upgrades, there's less to loot.