You've got an iron head 617. Not nearly as finicky with temps as the Al headed 603 motors.

Anywhere between 80 and 100 is OK. Most usually run in the 85C range at all times.

If you want more constant temps:

Step #1: Replace radiator cap
Step #2: Check fan clutch
Step #3: Check/change thermostat and flush coolant

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
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