When I was a mechanic back in the 80’s almost every manufacture I knew of, foreign or domestic warned against using ammonia or vinegar in their washer fluid reservoir.
Since, I no longer want to spend my days getting salt water dripped into my eyes when I did a brake job in the winter, so I’ve (several times) changed occupation.
Sure I think that most (at least for warm weather) windshield fluids, you’re paying for shipping 95% water. I mix my own in the summer, with a bit of isopropyl alcohol for biocide, but I always use commercial fluid in the winter because at the consumer level methanol isn’t that cheap (esp to achieve the small amount needed for the antifreeze properties).
My question is that people, in fact the vast majorities of internet entries on making your own wiper fluid (Windscreen water infection risk - BBC News aside) use some sort of ammonia. Now my current car (2008 Honda CR-V) says no ammonia, but have washer systems upgraded over the years that most of them can take ammonia? I always thought that the problem is that washer fluid just sort of sits there most of the time and slowly does whatever it’s going to do.