The oil market should function differently. Oil should have a fluxuating price based on demand. It shouldn't be set by each individual oiler trying to undercut another. Oil shouldn't be bought in the market by each player either. It should be obtained automatically each turn based on a percentage of money that the country is willing to spend on it.

On top of that oil should have an impact on countries beyond just attacking. Such as on a country's economy. Without sufficient oil a countries economy should suffer. Say you need 1/1,000 barrels per population to break even on your normal cash flow. If it drops below that tax revenue decreases steadily. If you buy more than that you stockpile oil for military usage.