The Big Oil job will become unlocked on your map after reaching Level 30. It is recommended you complete Day 2 stealthily, as it gives you enough time to look for all the clues in the lab. The clues will help you in identifying the correct engine. The correct engine and clues change each time you replay the mission. The clues are as follows:

1. There is a computer screen with a PSI value in the top right corner. It will tell you whether the engine has more than 5812 PSI or less than 5812 PSI. 5812 PSI are equal to 400 bars. There is a little white watch at each of the engines, allowing you to see how many bars the engine has.

2. There is a small clipboard on one of the tables (random location) with some black text written on it. It will tell you how many cables the correct engine has. It either says 1H (1 cable), 2H (2 cables) or 3H (3 cables). These are the cables going from the top of the big blue bottle to the engine itself.

3. There is another clipboard with a green checkmark. It will tell you whether the engine uses Nitrogen, Deterium, or Helium. View the large white board in the entry area of the lab to see which substance has which bottle color. You can then look for the small, colored bottles inside the engines (orange, blue, or green).

Use all of this information to narrow down the possibilities. Once you are sure you have found the right engine, pick it up, and bring it back to the airstrip. Activate the red flair, and wait for the helicopter. If you did this stealthily, the alarm will now go off automatically. Throw the bag with the engine into the helicopter, and survive for another three minutes until you hear back about the test results. If the tests confirm that you picked the right engine, you will get the "Doctor Fantastic" achievement (before completing the mission).