Begin a level and then save the game.

Use hex editor... In the Survivors levels the first line of your hex
will look something like this:-
000000 0100 0000 0000 0000 0000 0000 0000 0000 "

Regardless of the numbers that may be in your saved game change this to:
000000 0100 0000 0000 0000 F301 F301 F301 0000

This will give you over 32 million oil credits.

In the Mutants levels your hex will look something like this:-
000000 0200 0000 0000 0000 0000 0000 0000 0000

Change this to:
000000 0200 0000 0000 0000 0000 FFFF FFFF FFFF

This will give you a negative oil figure with the result that you
will never run out of oil credits.