Oil and ammo are unit-support resources. Oil and ammo are displayed in the stats bar alongside money. Note that these resources display only your per-minute income; oil and ammo are never stockpiled the way money is. Whenever you deploy a unit or construct bunkers, they draw from your income of ammo and/or oil. This is known as upkeep. If, for instance, you have an oil income of +20 and deploy a heavy tank regiment that has an oil upkeep of -18 oil, your oil income drops down to +2 as long as that tank regiment exists. If you should use more resources than you bring in, the difference is deducted from your monetary income. Buying extra supplies isn’t cheap. One unit of ammo costs two units of money, and one unit of oil costs three units of money. If you have an oil income of +10 and build the same heavy tank regiment mentioned above (-18 oil upkeep), your oil income drops to -8 per minute. That -8 oil income would cost you -24 units of money per minute an expensive problem indeed!
Source: Axis and Allies Official Manual
ONE OF THE MOST COMMON MISTAKES NEWER PLAYERS MAKE IS TO DEPLOY OIL AND AMMO DEPOTS INSTEAD OF SUPPLY DEPOTS. WHY NOT MAKE OIL and AMMO DEPOTS? THE ANSWER IS TWO-FOLD. MOST IMPORTANTLY, OIL and AMMO DEPOTS BOTH COUNT AGAINST YOUR BUILDING LIMIT OF 20, WHEREAS SUPPLY DEPOTS DO NOT. USE YOUR 20 BUILDINGS WISELY, THE MORE HQ's, THE BETTER YOUR ECONOMY. SECONDLY, YOU GET MORE RESOURCES FOR YOUR INVESTMENT USING SUPPLY DEPOTS.
