Found a reference to issues I read about before.
Could all be utter bunkum, but makes sense-kind of.
"I don't know about the rules for fluids in the UK, but in the USA, you would NOT want to use a DOT5.1 fluid in a street car. The reason being operating temperature. A DOT3 and DOT4 are both rated for street use, that is they have the correct operating temperature for street use. They have a relatively low boiling point so they will still flow well in ambient and even cold temperatures like in winter. A DOT5.1 is more of a race fluid, which means it's a thicker viscosity. This helps raise the boiling point, but because of how much thicker it is, it can be somewhat spongy or hard to press because it will, especially in cold temperatures (0-5ºC or colder), have a consistency like honey, rather than water. Imagine pushing the pedal and trying to move the brakes with honey in your lines. Because brake fluid gets very warm during racing, sometimes upwards of 300 degrees, it needs a higher boiling point. But it also needs to flow the best at the normal temperatures it will see, so that's why it's formulated that way."