There are two things you have to be careful of.

The first is using something that will chemically attack any protection system you may have.

The second is to avoid things that lower the surface tension of the water AND that have some further effect on the paint or metal.

When using washes, they must be pH neutral, PARTICULARLY if you spray it on with a pressure cleaner. These liquids generally lower the surface tension of the water and the pressure cleaner will force the liquid into gaps and seams that even normal water under pressure cannot go. As a result, the liquid that cannot get out will start the corrosion process.

As a general rule, big name windscreen washer additives should be safe, however you have to be careful because they can deteriorate with age. Many products should have a use by date as what starts of pH neutral can either become acidic or caustic with time.