On day 2 of the Big Oil heist the crew is required to steal a fusion engine prototype. In the facility there are multiple prototypes, how do I determine which engine to take as it is time consuming to check each and every one of them?
On the second day in Big Oil, there are 3 hints that can be found on the map.
The first hint is on the computers in the lab, one of the computers will have an inequality symbol (≤, ≥, <, or >) and a value in PSI. You must take that number and multiply it by 0.07 (approximate, accuracy really doesn't matter too much). This will give you the reading to look for on the pressure dial of each engine. The PSI value seems to be close to 400 BAR every time, so it is more important to look for the inequality symbol.
The second and third hints are found on clipboards around the house. One will list the type of gas used, Nitrogen, Deuterium or Helium. Each gas will be a different colour, as seen on a whiteboard in the lab: yellow (Nitrogen), blue (Deuterium, spelled incorrectly on the whiteboard), and green (Helium). You must find the clipboard, match the colour, and find that colour tank on the engine.
The last hint is the number of cables connected to the large blue (hydrogen) tank. The clipboard for this hint will list the CAS ID for hydrogen, molecular composition, molar weight, and other details, but what you are looking for will show up slightly larger and at the bottom. It will either be "H", "2xH", or "3xH". You match the multiplier to the number of hoses attached.
Now, not all the hints will be present in any given instance of Big Oil, so you may have several potential candidates, tough luck. You will have to guess at one or a few and bring them all to the helicopter.
One last note, not entirely related, but the intel you can retrieve on day 1 is not be particularly useful to solving the puzzle, but may make life easier for the team. The most useful intels are the airplane keys to provide an alternate, no waiting escape; a keycard, to get into one of the potential server rooms without drilling/jamming/ECM; and the shutter codes, which are used to open the shutter in the server room, if present, and to close the outside shutters for greater cover if/when you alert the police. Other possible intels include exterior blueprints of the house (doesn't change from heist to heist, doesn't list server room), a page of research (could be relevant, no idea), and pictures of the guards or the scientist (if you can't tell what a person looks like, you shouldn't be playing payday, let alone any other game; the scientist wears a lab coat, go figure).
Good luck, go get paid.