If I read it right, you're temps went up ~ 15ºC when you switched from plain water to an ~ 50/50 water/MB coolant mix and nothing else?

Plain water is better at heat transfer, but, that's a big difference.

I'd suspect the coolant system isn't "burped". Run the heater, disconnect the top rad hose and whatever else you may find in "search" using or as a keyword. FWIW

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)