I would suggest that you install the correct thermostat. Without it you don't warm up the engine fast enough. If your engine is running too cool, it's not healthy for the engine. You don't boil the moisture and acid out of the oil, and there is more wear in a cool engine than a hot one. I have measured cylinders in a few engines before and found that the end cylinders which run cooler because they don't share the coolant with the neighboring cylinder on both sides, have more wear. The hotter cylinders will last longer. There are limits to this of course, they should not be TOO hot either.
Thermostats are inexpensive and easy to replace.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in