IMO, for automotive use, distilled and DI water are interchangeable. Pure water, however you make it, immediately begins forming ions from the air (CO2 in particular), and once it's mixed with antifreeze, it's all the same. My understanding is that both processes are very good at stripping the water of ionic species, but each will have certain types of impurities unless additional processes are used to address them. From what I've seen, a typical DI system will first filter the municipal water to remove particulates, then run it through 3 or more ion-exchange beds, followed by a second particulate filter (the filters aren't 100% efficient). This water is suitable for many industrial chemical processes, and I have no concern with using it to top off my expansion tank. However, absent additional processing, some microbes can survive your typical DI filtration system. This is where distilled water is sometimes preferred - a properly designed still can produce sterile water quite easily. This, for example, can help to prevent biological fouling of a cooling system. In terms of corrosivity, IME (and from what I've read), there's not much difference. Highly purified water, no matter how it's made, can be corrosive under certain conditions. But corrosion is not typically a problem - cooling systems that employ pure water were designed for it, and glycol-based coolants are generally formulated with corrosion inhibitors. There are many different cooling systems out there, and coolants are customized for them. The corrosion inhibitors and other additives are selected based on the specific design, materials of construction, servicing requirements, etc. I'm not a chemist, or a cooling system engineer, so FWIW, and
IME.
