I know this is old....but felt this should be added:
DOT; 3_4_5.1 are all glycol base fluids.
DOT 5 is a silicone base and should not be mixed with those others.
Other brake fluids with higher boiling points will state which fluids they are compatible with. But to gain their full advantage, you should drain the system completely.
Here is one chart on boiling points...and to show why you should drain and replace brake fluid yearly.
Wet Boiling Point is when there is 3.7% water within the fluid...and this may occur at near two years. There can be near 2% water in fluid in the brake system in just one year. Water can be nearing 3% in just 18 months. Some fluid, like Brembo LCF 600 Plus has boiling points of 601°F dry__399°F wet__and is compatible with DOT 3_4_5.1. But again, a system should be completely drained so to not lower the performance of say a Brembo LCF 600 Plus.