In the past few years I have seen an increase in use of newer commercial de icing mixes on highways (brine slurry and eco friendly mixes) as well as non petroleum based chemicals used by commercial road construction.

The result is the type of condition you have indicated.

I use filtered water and P-21 Washer concentrate with 15% commercial windshield washer fluid and that seems to work with the heated washer fluid compartment MB has as OEM.

I have noticed that most commercial windshield wash fuid gets "gooey" below 40 deg F (You will see a visible smear behind the wiper blade).

I don;t use Rain X as I grew tired of the chattering blade syndrome and the resulting damage to the Wiper motor system on both my MB and Volvo.