Recommended coolant type

Regular water does not have anti corrosion additives so absolutely should be avoided for use more than a few weeks at most IMHO.

Evans is a high quality propylene glycol, so it can be used without any water. Its cooling properties are not quite as good as a water/anti-freeze mix; Evans makes no bones about this point.

For cost effectiveness, a 50-50 AF/water mix is still a good buy.

As for corrosion and other problems, 2 things are likely:

• Old AF loses its anti-corrosion additives after several years. If not changed out, that will cause AL corrosion. If the AF is kept refreshed, then there will be no AL corrosion problems.
• Dexcool will 'gunk' up if any air is entrained in the coolant. This can be caused by boiling, from running a non-pressurized system or just too hot too often, or from cavitation from water pumps or other design matters in the cooling system that promote cavitation.

IIRC, every engine mfr already has re-designed engines these days to avoid cavitation problems so they can run Dexcool-like AF. This is to get to 10 year changeout intervals with a Dexcool/water mix.