Cheat mode:
     While  in  the game hold L1 + R1 + R2 + L2, press Start to access
the  pause  menu. Select display password. While viewing the password,
press  Left, Circle, Square, Right and X. This will allow you to acess
invincibility,  50000 oil and full map at the new menu (cheats) on the
pause menu.

Passwords:

KQULLZ - Start on last mission with The Evolved 
GTH6JZ - Start on last mission with The Surivors 
MHICYZ - Start on mission 7 with The Series 9