What's the proper washer fluid additive that makes it kind of soapy?

Long stroy shot, I finally got my wiper act together. For several years I've put on new wiper assemblies (you know, the whole clip on thing). They were the Bosch "Mercedes quality" replacements. They worked OK but were just passable. I had saved the original wiper assemblies (stamped with 124 and 202 part numbers) from years ago. I finally wised up and ordered the replacement rubber for the orginal assemblies (000-824-36-27 for both 124 and 202) and man what a difference. This wiper set up rocks. It took me most of a bottle of wine to figure out how to slide those suckers in there, but dang they work GREAT.

So... what's the proper additive that make the washer fluid kind of soapy?

Local dealer had a one or two ounce bottle for $5 that looked like soapy water. I asked the gentleman at the parts counter how many drops per gallon of washer fluid. He said to dump in the whole thing. I had to laugh. I use about two gallons of wiper fluid a month. That's ten bucks for soap! Is there a "proper Mercedes" "soapy" wiper fluid alternative? What am I missing here?

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Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230