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.
God made the integers; all else is the work of Man. -- Kronecker