Can I Believe These Oil Temps?

One additional piece of data.  The oil temp sensor I recently put in is what everybody says is equivalent to the Rotax part:
https://www.aircraftspruce.com/catalog/inpages/flightline13-16029.php

The official Rotax part is 9x the price: https://www.leadingedgeairfoils.com/cht-oil-temp-sender-for-rotax-reg-912-914.html

Why such a huge difference in price?  Is the cheaper one as trustable?

Go with the Aircraft Spruce offering, at 1/8 the cost of the Rotax/leading edge rip off. 

What's the worst case scenario - doesn't change your oil temperature reading?

I already bought and installed the Aircraft Spruce sensor, so it’s brand new. 

I wish I could resolve this…

I would bet this is a gauge problem or wiring to the gauge. It seems unlikely to get temperature swings like that, or for the oil to run that cool overall even with the oil thermostat wide open.  What display are you using for the oil temp readout?  What water temperatures are you seeing?

You should be able to do something as simple as warming up the plane on the ground, then check the oil filter or oil tank external temperature with an infrared thermometer. You could also check the external temp of the oil temp sender. It won’t be perfect but it will help determine if your gauge is close.  

With an ambient air temperature. of 40-5o*F, the oil temperature is not going to change more than a few degrees per minute. The oil is not going to cool off 40*F in less than one minute. Your sensor is new. My guess is something is wrong with the wiring, the EIS or the settings in the EIS are not correct for the sensor you have.

Full disclosure - I am new to the Rotax world, but have decades of experience with oil temperature, both engine and hydraulic. Oil temperature does not change as quickly as water temperature.