I use the MB concentrate, and you do use the entire bottle of it per gallon of store-bought washer fluid. I think it makes a big difference, but my dealer sure doesn't charge $5 for it. I think I pay around $2 for them and buy a handful at a time. I also don't get the cheap blue crap, I prefer the Prestone de-icer. Connecticut winters get pretty sloppy, but I no longer have any windshield problems. It's a few dollars well-spent.

Tried RainX - hated it. Others love it; matter of opinion I reckon.

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2002 E320 4-Matic
2008 Subaru Outback
2009 Subaru Forester