That Adafruit display would be nice - if it fit. The Adafruit display window is 1.1" high and there is a bezel around the window and there is a backing circuit board which is yet a bit larger. I had a look at my clock and I am thinking no-go if you want to fit it behind the existing clock opening. You will need to pull the clock apart and measure the window dimensions and space behind the window to assist in selecting a display.
I thought the Teensy 2.0 was out of production; but, I see that it is still available from PJRC. The 2.0 would have the processor speed and memory to do the job just fine. Its only limitation might be the number of I/O pins if you decided that you wanted to go big and display more than oil temperature (you are sure you don't want to do AFR?). The one 'ick' factor is that the Teensy lives on 5 volts which means that you will need a small power supply to provide it with 5 volts. Not a significant problem; but, it does mean a separate little piece to deal with. Some of the Arduino boards are tolerant of much higher supply voltages. I know that people have supplied Arduino boards directly off of the car 12 v system and the on board power supply on the Arduino has an external 5 volt bus which would work nicely to supply the reference voltage for the RTD. However, car 12 v systems can run 14 v or higher which is probably pushing your luck on the Arduino's 12v upper limit. Anyway, not a material impediment. Something like this would probably do the trick for power
https://www.amazon.com/Regulator-DROK-Converter-Step-Down-Transformer/dp/B0758ZTS61
and the power supply can supply the regulated pull-up voltage for the RTD.
Yes, the AEM sensor is just an RTD (resistive temperature device) whose resistance varies with the device temperature. You convert the variable resistance by connecting the RTD in series with a fixed resistance (the pull-up resistor) and then connecting the two resistor to a fixed voltage supply creating a simple voltage divider circuit when you measure the voltage across the RTD. The equation for Vout versus Rrtd is
Vout = (Vref)(Rrtd)/(Rfixed+ Rrtd)
At 0 C with the 2200 ohm pull up resistor and a 5 volt reference voltage the 37500 ohm resistance of the sensor will give you an output voltage of (5v)(37500)/(2200 + 37500) = 4.723 volts (just like AEM says). The RTD is non linear in its response so you can fiddle with the Rfixed a bit to give you better resolution in the temperature range that you are most concerned about.
You could do a look up table; but, that would be computationally more intense. Every time you do read you have to start at one end of the table with a line by line less than / greater than search to find the line with your voltage measurement and you voltage will never match the line values so you will inevitably have to interpolate between lines to get a temperature measurement. The much easier way to do this is to use the Steinhart - Hart equation that is referenced in that
Arduino link. The Steinhardt - Hart equation is looking for resistance in and gives you temperature out. If you massage the Vout equation above a bit you get:
Rrtd = (Vout)(Rfixed)/(Vref - Vout)
At 0C you measure a voltage of 4.723 volts directly across the RTD, plug that into the above equation and you get Rrtd = (4.723)(2200)/(5-4.723) = 37,499 ohms after a little rounding. You plug the calculated resistance into the Steinhart - Hart equation which gives you the temperature for that resistance value. Computationally it just requires two lines of code - easy peasy! You will have to populate the coefficients for the Steinhart - Hart equation which requires a tiny bit of number crunching (details below); but, you only need to do that once unless you change the sensor to an RTD with different characteristics.
https://en.wikipedia.org/wiki/Steinhart–Hart_equation
For the curves that describes the characteristic of an RTD, picking 3 data points from the AEM table will work. Picking one data point at each end and one in the middle of the range will work. If you want more accuracy in the 50C to 100C range you might pick 50C, 75C and 100C; but, then temperatures outside of that will likely have more errors as you get further below 5 C and further above 100C.
If you are visual, I can scratch down a crude single line diagram showing the connections. Most of the code to do a single temperature measurement already exists in that example you linked in your first post. The coding that remains will depend on the final display that you want to use.
If I were doing this on an ECU that also happened to be driving the display, I would put some filtering and protection on the input measuring the RTD voltage just to avoid electrical nasty stuff getting onto the controller board and possibly causing resets of the controller. Since this is not a mission critical device, I don't think those additional bits are really critical.
