Ok, I'll try not to ramble. On an old vehicle, you can put pretty much whatever you want in it. As long as you flush it out. You have a 98 chevy pickup, you want to put green in it to save a few bucks? Fine, but flush it good first. If you mix it with the factory dexcool, in a few months you will have what looks like chocolate pudding, and reduced coolant flow. Newer cars, not a good idea. If your new lexus calls for something specific, put it in there, even if its $100 a gallon. Because that $100 a gallon is probably the only thing that doesn't act like acid to a $1000 nasa space age metal alloy radiator or intake. It may look like plain ole aluminum, but don't bet on it.
So for that reason, you don't do it in a diesel either. 6.4's for example, have a habit of cavitating and eating out the front timing cover and dumping the coolant into the oil if the wrong coolant is used. Bad deal.
But on a diesel, the main reason is far more destructive. A diesel is a high compression motor. If you look at the last picture in FTG's post, you can see the rings around the tops of the cylinders. There are sleeves that go down through the block forming the cylinders, and surrounding the cylinders is the coolant. When the piston goes down, the turbo fills the cylinder full of air, the piston comes up, it compresses it as tight as it can. If its tight enough to stretch the headbolts (which is why you get blown head gaskets), its tight enough to "bulge" these sleeves. When the sleeves bulge out, it pushes the coolant out with it. When the piston goes back down and the valve opens, the sleeve relaxes. The problem that was occurring was the aftermarket coolant couldn't keep up. It was forming tiny bubbles, if you will. When the next cycle came around, the bubbles would pop, and the coolant would implode back against the sleeve. Over time this pressure washing effect would eat its way all the way through the sleeves and into the cylinders. Game over. The manufacturer specific antifreezes are formulated to "stick" to the sides of the cylinders to prevent this from happening.
I'm sure scientifically there are many other reasons, but these are the main, real world reasons for not doing so. Its just not worth the few pennies you save. I know that people do it all the time, and most of the time, it goes unnoticed. But all it takes is once. And if your the unlucky one, all of a sudden, its *$#&@ FORD! And its not necessarily Ford's fault
