I believe the thermostat for this car is set to open at 87 degrees Celsius. That is what is stamped on the body, and I have boiled a few in the kitchen with a thermometer in the pot just to check them.
Given the differences between one car and another, seeing the needle favor the 100 degree mark rather than the 80 degree mark is not unusual. But if the needle is on the 80 degree mark or below it after the oil temp is near 100 degrees C you have a worn out thermostat.
The "right on 80" thermostat is what I am familiar with from my Diesels, which typically run like they have an ivisible peg in the instrument on the dash that keeps them from going over 80 or 82 degrees C.
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)