Update:
Took the car out for about an hour today after wrapping the
oil cooler in metallized 3/8" ceramic wool thermal barrier. The air temp was in the low 60s. The water gauge read between 80C and 85C. The
oil temp gauge read a tad above 40C. When I got home I put an instant read thermocouple on the remote
oil filter mount and it read 180F (82C). That makes me feel better that everything (except the oil temp gauge) is working the way it should. I'm guessing either it's just a calibration issue with the bourdon tube, or the tube is cooling the liquid inside and messing up the reading. I can live with it being low, as long as the oil is not too cold.
I did hear back from Nissonger and they recommended checking calibration by pulling the bourdon tube bulb from the engine and submersing in boiling water. I may try this next time the car's on the lift. Or just live with it.