Car shampoo sounds simple, but there's more difference between products than you'd expect. pH value, foam power, scent, and concentration vary widely. For a regular hand wash you want a pH-neutral shampoo that provides enough lubrication to protect the paintwork and release dirt without stripping wax or coatings.

A good shampoo gives enough slip so the wash mitt glides over the paint rather than drags. How much you need per litre depends on concentration. Many professional shampoos are highly concentrated and cheaper per wash than they appear.