If your not going all the way to dot 5 synthetic use dot 4, has a higher boiling point but will absorb moisture over time just as dot3 will, so annual flushes still required. dot 5 full synthetic lasts virtually forever as it will not absorb moisture & cause a drop in boiling point but will require a full on flush. if you do convert flush the whole system with alcohol to be sure all the old stuff is out. dot 3/4 & dot 5 are not compatible & any remaining amount will cause a spongy pedal.