Here's a clip from an article although not speaking of Harleys , but Saab cars , gives you a overview of the uses of the different brake fluids and problems that can occur from mixing them.

Well worth the read, considering the problems you can have if you make the mistake.


A note about brake fluid standards:
The recommended brake fluid for the Saab 9000 is DOT 4 (or compatible). When I replaced the brake hoses on my 9000 Aero, I flushed the system with DOT 5.1 brake fluid and received a number of comments from US 9000 owners expressing concern that this would be incompatible with the braking system (to be fair, I did describe the fluid as "DOT 5"). DOT 5 brake fluid is a silicone-based fluid and is not compatible with DOT 4. It is also less suitable than DOT 4 for ABS systems, because of its higher compressibility. I believe that the DOT 5 standard is not recognised in many European countries, hence the widespread use of the term "DOT 5" as shorthand to describe fluid that actually conforms to DOT 5.1.

DOT 5.1 brake fluid is a low-viscosity, high-temperature fluid compatible with DOT 4 and DOT 3. Its low viscosity makes it ideally suited for ABS systems, where it helps the system modulate the braking pressure more easily. Its higher wet and dry boiling points should make it more resistant to brake fade under heavy use. I have had no problems with the braking system since installing this fluid.

DOT 5 and DOT 5.1 are very different standards, confusingly named by the US Department Of Transport. Of the two, only DOT 5.1 is compatible with DOT 4. Do not use DOT 5 fluid unless you have a good reason and know how to purge the system of all DOT 4 fluid beforehand (this usually requires complete dismantling of the braking system and cleaning of the individual components with a suitable solvent). Mixing DOT 5 fluid with any traces of DOT 4 will form precipitants that will clog the braking system.


Source: http://www.saab9000.com/procedures/b...rakebleed.html