RAF Bomber Command began an offensive against the German oil industry in 1940. It became it's main target in early 1941. The oil campaign was abandoned in July 1941 because post raid recce showed that little if any damage had been done to the targets.
In 1940, about 15% of BC tonnage went on oil targets, which is about 15 times as much as they expended on area bombing (their main job in 1940 was anti invasion work, and the Battle of France)
The simple truth is that the RAF could not bomb oil targets in 1940, 1941, 1942 or 1943. The accuracy simply wasn't there. It wasn't until the development of better targetting aids, which were in widespread use by 1944, that the RAF was capable of bombing oil successfully.
If the oil offensive was started in spring 1943, then Germans would have been in serious trouble by early 1944.
Whilst the RAF didn't have the accuracy to go after oil by night in 1943, the USAAF didn't have the numbers, or the escorts, to do so in 1943. Oil targets frequently meant deep penetrations which were suicide without escort, and required large numbers to do significant damage. The USAAF was still a small bombing force in Europe in 1943.
The Bomber Command should have been used more directly against the war machine, than being wasted against German cities.
It was. Once the accuracy to hit small targets was available in 1944, BC moved from area bombing German cities (which was about all it could do in 1943) to attacking a large variety of German targets, from specific factories to oil installations, troop concentrations to transpot targets. In 1943 BC dropped about 90% of it's bombs on German cities. In 1944 and 1945 that dropped to about a third, with the rest being expended on various specific targets, not area bombing.
He was so tied up on bombing cities he missed the key link to the German war machine, it's oil. Bomber Command should have been thrown at the German oil production facilities at full strength, but instead they were sent out against the cities in a waste of steel, lives and bombs.
What a lot of people don't realise is that BC bombed oil targets heavily in 1944 and 1945. In fact, they dropped far more bombs on oil targets than 8th AF did. It's only when you count in the USAAF forces in the Med that they dropped more bombs on oil than the RAF.
Tons of bombs dropped on oil targets:
RAF BC: 97,914
8th AF: 60,800
USAAF Med: 51,860