I am doing an engine swap, a newer engine into an older body. The older car was carbureted, had a mechanical fuel pump and all that stuff. New engine is meant to be fuel injected, needs electric fuel pump. The old car has no means to control a fuel pump. I was looking at an arduino to run the fuel pump based on the oil pressure signal, that way if the car is involved in an accident, the engine will stop and the pump with it preventing a fire. The Oil Pressure sensor uses resistance, so it would read the resistance to determine to run the pump.
The idea is that it primes the system, then cuts off without oil pressure, and stays on with oil pressure.
I also want to be able to control the cooling fan since a mechanical fan will not fit. Read off the the temperature sensor and turn the fan on when the engine gets to a certain temperature.
For the fuel pump cutoff, it would likely be more reliable to use a tee connector at the oil pressure sensor and an oil pressure switch - there are pressure switches made specifically for this use.
I would suggest before spending a lot of energy on this you need to read the ST application note AN2689 and others like it, It will start to give you an idea of what is involved. There are several regulations covering what you are doing, but it is dependent on the country as to what is allowed and what is not.
There are oil pressure sending units and simple oil pressure switches. Most single pole switches are a single pole to ground and some have two leads acting as a switch between the leads. Most switches activate between 3 to 10 PSI. All you really need is a switch, there is oil pressure is True and no oil pressure is False, just write your code around what you have. Using an Arduino for this is way overkill. Most oil pressure switches are very capable of driving / switching an automotive relay. The relay in turn powers the pump.
Even if you wanted to add measuring coolant temperature a simple circuit module could be employed.