First off, make sure you have your hired Jet Pilot.

Once you do, take his Airstrike Challenge (I'm at the final challenge, so it may require that you get to the final challenge).

During the challenge, you will be using laser guided bombs. For some reason, after you have completed the challenge, lose or win, you will have unlimited laser guided bombs with no oil cost!

This will last until you leave the game, save or not.

Afterwards, you will have to do the challenge again in order to get unlimited.

This is a good way to get reid of pesky buildings from afar without getting hurt. It only works with laser guided bombs so far, but its better than spending your oil up quickly.