The market economy, based on deep human psychological propensities, is an extraordinarily effective societal instrumentality for planning and coordinating the production and distribution of scarce, rival, excludable commodities.
I also played through Beyond Good and Evil again not so long ago.
If you like Beyond Good & Evil, you should try Enslaved: Odyssey to the West, which was ported to Windows in October. I really enjoyed both.
Petrol may well be more valuable than gold in a Mad Max scenario, but don't think for a moment that any collection of humans will not want a way to disintermediate their labor and have a means of trade.
Within their society they'd use reputation-based credit, and with outsiders they'd use petrol. It's not clear why they'd need gold at all in your example.
Live within your income, even if you have to borrow to do so. -- Josh Billings