I can't help with the oil gauge mystery I've never heard of such a thing happening but I can help with the temp thing. Take the sending unit out of the car and put it in a container of water. Use a container that can be heated. Put a digital thermometer in the water. These can be bought at Wally World from the kitchen supply section. Put the sending unit in the container and reconnect it to the gauge. Also run a ground wire to the car body or negative post on the battery and clip it to the threads on the sender. Then heat the water and compare the gauge reading to the thermometer reading. I've used this method for years working with boilers and chillers. Its fool proof.

The stock temperature sending units should read about 75 ohms when at room temperature, about 10 ohms when hot, somewhere in between for normal engine temperature (around the boiling point of water in open air).

Have you changed your oil pan or dipstick?  If you have you may have less oil in the engine that you think you do.  If you've changed the dipstick put in 4 quarts of oil and make sure that the oil level and the "full" mark match.  If you've changed the pan, did you also change the pickup and ensure that it's the correct distance from the bottom of the pan?  On the first fill did you also add the amount of oil matching the pan capacity, then check the dipstick to confirm that the oil level shows correctly on the dipstick?

Thanks also for the tip on the temperature sending switch!