|
DOT3, DOT4, and DOT5.1 are all generally compatible within a braking system, as all are all glycol-based brake fluids.
DOT4 & DOT5.1 tends to have higher temperature resistance than DOT3 for performance driving. However that temperature resistance comes with the price of higher hygroscopy - they tend to absorb more water from air compared to DOT3. This essentially degrades their performance more quickly and means they need to be changed more often to maintain their characteristics.
--Most people who use high performance fluids in HPDE/race settings don't encounter this issue as they change the fluid more often than the standard service interval just due to the rigors and demands on a road course
Generally avoiding mixing has less to do with straight compatibility/damage to the system. I think it has to do with maintaining the predictability of the braking system over time, as the performance level and drop-off of the types of fluids change at different rates. Starting from DOT4 and mixing in DOT3 means you're lowering the effective boiling point. If a driver is expecting DOT4 performance, they are at risk of an incident if it is not delivered.
|