Oil production in Germany was on the rise until refineries became the target of Strat Bombing. I will get you the numbers. The general problem with bombing factories is that you have to hit tools with a bomb. If you just hit the building the tools might get some damage but can easily be fixed. This translates not into hitting a factory but hitting a tool, some thing that might present anywhere from 5-300sq ft target. Luck would be the only reason for a hit in these cases as the US could only get 5-10% of thier bombs close to the target. Oil however is a much easier target becasue it is in the buildings and likes to burn and blow up. hit a tank and the whole factory is down for the count.