I have to wonder if heating is a coolent problem... If the system is clean inside most any of the coolents today will work, (IMHO)... One thing that will often help a system is putting a cleaner thru it.. some times they get traces of oils in them.. That will make them run warmer..
But besides driving old Mercedes from the mid 50s I am also a motorcycle fan.. And own and run several.. They get far more miles than the cars do.. And most of todays up scale motorcycles are water cooled... One thing that I have learned about Coolent is that the green stuff.. has silica in it.... Thats a fine sand so to say.. It helps polish the inside and keep it clean.. No I can not feel it.. The longer life stuff.. DEX-COOL... Does not.. Thats the pink stuff... Any way.. Seems on motorcycle engines the Silica eats the seals .. Most seals these days on water pumps are carbon.. with springs to hold them together until they wear out.. They run in the coolent to keep them cool.. Long gone are the old rubber seals .... Even on the replacement pumps in my old 123 Diesels.. I would bet that the Mercedes Coolent has no Silica and is low PH like the ones General motors insist on in their newer cars.. Water pump seals were an issue before starting to use DEX-Cool some years back in my old cars and bikes.. Now they seem to last as long as the bearings in them hold up and never leak.. The inside of the system seems to show no difference.. Still very clean.. I change at 2 or 3 years even if the new stuff is 5 year or so stuff..
In case you can not get Mercedes Coolent or do not wish to buy it...I would lean to getting the Dex-Cool (Pink stuff) made by most any coolent company and kind of stay away from the green stuff.. Its not bad stuff.. Just seems a lot harder on the water pump seals and the plastic parts used in cooling systems these days... I have also noticed that I have lots fewer problems with the small pump that runs the heater water leaking.. They too have the carbon seal..
I still see no difference in cooling as far as whitch kind it is.. Just so its the right amount and system is clean (No oil in system.. They all need cleaning from time to time) and pumps work right.. belts right.. fan clutch works right.. Electric fans come on when needed.. Hoses do not kink..