So DOT5.1 is compatible with Etylene-Glycol DOT 3 & 4 Fluids...where do you get it, is it color coded or by virtue it is not purple it is amber colored to imply it is "mixable". Do I take 5.1 implies that it has even higher boiling point than DOT4, but is not Silicone based like straight DOT5 "Purple"? When using DOT 4 I would use Castrol in the big bottle, as it was used on my truck & car, to flush all the systems every 3 years.
It should be available in most auto stores and yes it is compatible with 3 & 4. Couldn't tell you about the reason for the color of the fluid.