The 300SD runs at 85C. On the rare occasions I can go faster than 70mph on the freeway, it creeps up slowly. If I try to sustain 80-85mph it creeps to 90; I haven't gone fast enough long enough to see if it keeps going up.

The E300 Diesel stay rock-steady at 85C. (I was going to say it was "Like a rock" but that's another brand and another forum...

None of the cars exhibit variation with outside air temperature.

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'96 E300D 60k mi (wife's daily ride)
'95 Audi 90 120k mi
'92 GMC Suburban 139k mi
'85 300SD 234k mi (my daily ride)