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.

There should always be oil automatically in the market as well (to prevent potential problems) the price of it should just be high until oil is infused into the market by oilers (who would set a percentage of their oil to place on the market per turn). So the more oil put into the market the lower the price of oil gets (though it should decrease steadily).

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 country's 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.