Romania in the mid 90's was producing approximately 135,000 barrels per day, or about 27,000,000 litres.
http://www.indexmundi.com/energy.asp...aph=production
Ploesti is a refinery rather than a field and according to wiki
http://en.wikipedia.org/wiki/Petroch...try_in_Romania refines approximately 3,200 cubic metres of oil per day. That's 3,200,000 litres
per day.
At 1% of prewar figures (as we're told in the books) that's still 32,000 litres per day, or close to a million litres a month.
Note that during refinery (a process I don't understand 100%) you can actually get more volume out than you put in!
We also know that an entire months production was dedicated to shifting a couple of divisions. Using my earlier example of about 250,000 litres for the 124th, I think we can agree that the 4th GTA had more than enough to keep them going for weeks of activity.