[asperformance]

dot3, dot 4 and dot5.1 b/fluids all mix no problem but your better off flushing thro with new at least every 2yrs and potentially as much as every outing dependant on use/driving style

the numbers get confusing as some fluids are "only" dot4 but have far higher spec than dot5.1's (specifically designed for certain ABS system criteria)

as with most things not all fluids are equal and we would generally use Motul dot5.1 as a min. being one of the best performing of its type, then either Motul RBF600 or Millers Racing 300+ and finally top of the tree is the Performance Frictions RH665 b/fluid (which we now use in preference even over Castrol SRF)