This topic is very interesting. Mainly because the German “Wehrmacht” hat great difficulties with fuel shortages.
I would say, as long as you are not in control of at least one oil field your tanks can not “blitz” and aircraft can only fly defensive missions (or there should be a restriction how many spaces they can fly - for instance -1 for bombers and fighters)
There should be a rule which allows you to make a strategic bombardement of the oil fields.
That is a very interesting idea. I would like to hear more of it 🙂