Dot 5 is silicone, and because it is silicone it absorbs NO water, unlike hydroscopic fluids which are 4 and 5.1.
The problem with silcone is that it isnt that great and water does get into the system, but instead of being homegenised through allt he fluid, it groups together in droplets, and when the fluid boils, it turns to gas and puts gas into your braking system......which = zero brakes.