While most of it's rumor, there is some talk that if your washers lines ever clog up your warranty won't be covered unless your using the right fluids.
The Volkswagen washer fluid isn't exactly cheap at $10/L, but then I don't know that there are many parts that are. Since we seem to tear through the stuff I thought I would look a little closer at it. The text on the bottle says:
"Suitable for use all year round for the windscreen washing system. Offers the necessary protection against frost, stops the jets from freezing up, frees the windscreen from dirt and protects both paint and the wiper blades. Can be used in all models, must be used for fan-type jets."
It's listed as being mixed at 1 part fluid to 4 parts water and since our reservoirs are 5L, that's a full bottle to fill it up from empty... or so it would seem.
Last time around I thought I would measure how much I was putting in. The light had come on early in the week and it was continued to be used throughout the week then filled on the weekend. It seems that after putting in 1L of fluid I only got in another 2.5L of water for a total of 3.5L so that light comes on pretty damn early.
I haven't tested running it till it doesn't work anymore to see how low it gets since I assume that most people would want to get rid of that light and re-fill the fluid.
So onto some maths (and you thought you would never need it again). Assuming you started with the right concentration you now have 0.3L of washer fluid and 1.2L of water in the tank. Adding in the full bottle of washer fluid every time will see you end up with a concentration almost double the requirement of about 1 part fluid to 2.5 parts water. This means your good down to almost -16C!
So to keep it 100% on what the documentation says you would need to add in 0.7L and top up with 2.8L water. Saving about 30% on fluid right there (although you then have to measure out the right amount each time. Something in my brain doesn't like this since you keep ending up with 300ml left in the bottom of a bottle...
Plan B would be to get a 5L container and pre-mix it then just top up as needed (although I don't know how well it stores in this fashion).
Plan C is to question the need for that concentration. I note that the various ratio's are all about temps below zero, the second to last one being -16C. I'm guessing a large part of the concentration is about not freezing. Being in SW Sydney I rarely see anything below 0C so freezing in the lines isn't a significant danger.
Moving to a nice neat half bottle (0.5L) per re-fill keeps my OCD in check and means your running at about 66% the listed strength or about 1 part fluid to 6 parts water.
Now maybe I just spend all together too much time thinking about these things. Maybe I just keep pouring a whole bottle in each time I fill up and not worry about it too much since I'm saving about the cost of a cup of coffee at work. That and it's only refilled once every 6 weeks or so anyway

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