Each unit built should cost a number of oil tokens: Hence you cant just buy a bunch of battleships to sail around the world, you need fuel
every ten IPC worth of land units built cost 1 oil token
every ten IPC worth of air units cost 2 oil tokens
every ten IPC worth of naval units cost 3 oil tokens
each oil token must be bought before and can be bombed like a factory.
fractions are rounded up, so if you buy 11 IPC worth of land units you spent 2 OIL tokens
Thats battleship will cost you 6 oil tokens to make, so it really costs 26 IPC
The game has way too many IPC and too many pieces, stuff should cost more.
an oil center token costs 3 IPC, oil tokens are chips placed under counter costing 1 IPC each.
Every time you build crap you also spend oil tokens. If the enemy bombed them you cant build any units, so protect them.
You may also steal enemy tokens by conquest.