As I understand it (and I am more an aircraft guy than a automotive guy) DOT 3 is a Glycol Ether-based fluid whereas 5606 is hydrocarbon/oil based.
DOT3 is soluble in water -- 5606 not much.
DOT3 clear or amber -- 5606 is red
Viscosity of DOT3 is close to that of 5606 -- 1/4 the viscosity of...
I recently had a problem in an airplane with the brakes releasing slowly -- this squared off a couple of tires (on touchdown) before it came to me. New pads and new disks had recently been installed, so I assumed the problem was in the master cylinder(s). When I took the feed hose off the first...