An engine that's running too cold will suffer increased engine wear, deposits and emissions. In addition,
oil is also thicker when it's cool, so it needs to warm up to provide proper lubrication.
Oil also needs to get hot enough to vapourize any moisture it collects. Here's a good article, including a chart of engine wear vs. temperature, that may have people yanking their 160 deg. thermostats in favour of 180 deg. ones:
HOTRODSRJ’s COOLING TIPS Operating temperature vs power and longevity!
If you're running EFI, you absolutely need to run the appropriate thermostat, or the temperature sensor(s) will read a cool engine and adjust the fuel/air mixture to compensate. It may even adjust timing - but I don't know.
Even if you're running a carburetor, the engine will take longer to get to proper operating temperature w/o a thermostat - if it gets there at all. Before I put the thermostat in mine (180 degree F high flow thermostat) I found the engine was slow to warm up. Worse, when the temperatures dipped this fall the coolant temperature was peaking around 160-165 deg. F on the highway.
I don't know what my
oil temperature is, but my coolant temperature seems to be running in the correct range. I may be swapping out my oil pan for a Canton pan (15-766), and it has provision for an oil temperature sensor. That would be an easy add, but finding a space on the dash for the gauge will be the challenge.