I shouldn't be amazed, but everytime someone says not to use distilled (or
de-ionized) water in a car's cooling system I have to wonder what the heck
they are talking about.

You should use pure water (distilled, double distilled, reverse osmosis,
whatever) in the cooling system if you have any doubts about the purity of
the water coming out of your domestic water supply. Some are blessed with
sweet soft water, with just a hint o' chlorine, others (me included) have
well water that is pratically saturated with calcium and magnessium
carbonates.

I would not use my water in anyone's cooling system. You should see the
precipitates left in a kettle after just one use.

So I use water from the lab, 18 MOhm water (purity described as a function
of its conductivity). Yes the same water that is sometimes, and amusingly
described as "hungry" or "extremely corrosive",

As soon as you mix your pure water with the coolant, you don't have pure
water anymore, but you know that you have not introduced any dissolved
substances that may precipate out in the cooling system.

Now you have a solution of ethlyene (or propylene) glycol, water, and a
whole witch's brew of various salts etc as buffers and corrosion inhibitors.

Alistair