Since I had some 5 lying aroun, I mixed it with some 3 that I also had lying around; mostly because I didn't have anything better to do at the moment. It was easy to tell the differance, as one was purple and one was clear. It didn't mix well. Not as bad as
oil and water. But it definatly clumped/clotted.
5 is also hydrophobic; it will not absorb water. All brake systems will get water in them from the atmosphere. If the fluid is hydrophobic, water will pool. When it gets to the caliper, it will boil and vaporize with moderate braking. That's a bad thing.
3, 4 and 5.1 are hydrophyllic, they absorb water. Since the mixture will be pretty much homogenous, it won't boil as easily. That's a good thing.
As for the seals, 5 won't destroy or eat anything. You can use the standard parts - including rubber parts - with 5. But, the makers of brake parts say that mixing the two will create a caustic substance. I'm not sure how that happens, since they don't mix well. I'v never actually tested this statement. But the people who make brake parts say not to do it, and that's good enough for me.