Just to clarify:
DOT 3 and DOT 4 fluids are compatible with DOT 5.1 - but are NOT compatible with silicone-based DOT 5. DOT 5 brake fluid is dyed purple to warn against accidental mixing.
The use of the 5.1 designation is really confusing.
5.1 is a lighter viscosity glycol-based fluid with that was developed for use in ABS systems that need to cycle on and off quickly. It really has no advantage over the DOT 4 fluid recommended by Ducati.