I tried the rain-x junk in my W126 once and had similar results. Smearing and skittering wiper blades. It also turns to sludge from the intense heat of the heated coolant reservoir. Never again!

I use the regular old blue stuff now and it works just fine. Sometimes in the warmer months I'll mix a batch of the "Krystall Klar" concentrate but its not for cold temps and I use it all up/flush it out before cold season.

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