To use or not use DOT5 fluid?

As many have said, DOT5 is NOT compatible with DOT3 or 4.
DOT5.1 is.
Why they chose to even use the 5 (for 5.1) designation is beyond me...
It's not silicone based, as DOT5 is...
DOT5, though having a high boiling point, is somewhat compressible, generally leading to a spongier brake feel.