DoT5 is an old fluid that doesn't really have a use any more. It was used in race/track cars because of the higher boiling point. Harley used to put it in their bikes to protect the paint in case of a spill; they quit doing that 6-8 years ago.
There are some down sides to the 5. It doesn't like water. So when water does get into the system, it tends to pool, and end up in the calipers. That's a bad thing, as it gets hot and turns to steam in the caliper. Systems with 5 need regular flushing to prevent that.
5 is also compressable. Most drivers wouldn't notice that. But on the track you would.
The reason for 5 was the higher boiling point. The only option you had back then was 1 and 2. Racers used 5 despite the down sides.
Now you have plenty of options, with no down sides. Plenty of glycol based fluids have a boiling point around 600* or higher. Not many drivers need more than that on a Cobra. If you do, you probably have a design flaw.
There's a "new" fluid on the market, DoT 5.1. Supposed to by
synthetic with the same properties of as glycol 4, , but have a high boiling point. I have not been able to find any of this. I use Wilwood fluid.
DoT5 DOES NOT mix with any other fluids. If you mix 4 and 5, it will clot in just a few minutes. I tried it. Most brake systems are cleaned and assembled with a glycol compatible fluid. If you want to use 5, you'll have to disassemble and clean with alcohol. Then re-assemble with 5.