Use of Anti-Seize
I went ahead and removed the old Temp switch and replaced it with the new one. No sign of prior coating on the threads, so I guess that answers my question.
I had suspected the sensor was bad--the main fan did not go on until about 110-115 degrees.
It now goes on at 100 degrees like it should. This should get rid of that nasty overheating at idle/in traffic problem.