I think "everyone" may be being somewhat imprecise. For the car to run at 80C the needle should be on the white line next to 80 on the gauge.

The next white line between 80 and 120 is 100C and the last line with the red on top of it is 120C. So for the car to be running at 90 to 95C the needle needs to be covering the top of the 8 in the 80C, to just about to leave the 8 altogether. If it reaches the unnumerated white line half way between the 80 and 120C you have reached 100C.

I have never seen a Mercedes that runs at 80C. Look at the cars in my signature. These are gassers and diesels on two different continents and they have always run with the needle in the 85C to 90C range under normal conditions.

My DD, also an 81 240D is currently running probably too cool. The needle is just above 80C so likely around 82. This is a recent development and I think likely because of a stuck thermostat which I will soon replace. I had the same symptom on my previous one which was fixed with similar results.

I have not climbed in the mountains as you have, where are you located? Anywhere near Las Cruces? But I drive around in Phoenix and with temps regularly above 112F ambient and would not be concerned in normal driving unless the temperature started to get towards the half way line, ie, 100C.

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S