The thermostat opens at about 190 F + or - or about 88 C. In normal use it should run about 190F to 200F whitch is about 93 C. with a little difference in gages and when the thermostat really opens.. If it runs much less than about 190 or about 88C when working on the road then you would need a new thermostat.. If it stays in that general area I would just forget about it.. All the Mercedes I have had in the last 45 years seem to run about there.. and climb some on hard pulls up the steep grades around here.. As long as they stay out of the red it does not seem to hirt them.. I would worry far more about an engine that run much cooler.. If it runs at about 80C or 176 F. or less you really need a new thermostat..

Ken