Oil is a substance and source of income in The Conquerors 3
Oil appears as black patches on water. When worked by an Oil Ship, it will produce an Oil Rig, giving the player $10 p/m. It is a very efficient source of income as one Oil Ship only costs $35, which means that $40 p/m can be gained for the price of one Nuclear Plant, which only produces $15 p/m. Additionally it is nearly half the price of a Power Plant (which costs $65) which only produces $10 p/m.
See also: Energy Crystal.