on my '94 S500 and it was not marked. It was partially stuck open!
Data says it supposed to be 185F (82C) but the replacement was marked 80C. I'd recommend that as a good replacement value.
Re temps and how-to-lower-them, read these threads. Good reading if nothing else. M119 Viscous Fan mod & problems with aux. fan resistor-mod
This thread 'attacks' the the 'dormant' viscous fan clutch:
High engine temps SOLVED!