Beginning in 2006, HD switched to a DOT 4 Brake Fluid. DOT 4 fluid (as well as DOT 3) is glycol based while DOT 5 is silicone based. DOT 4 is tolerant to higher temperatures, but is more susceptible to moisture contamination, as well as corrosive to paint. DOT 5 is not as tolerant to high temps, but will generally not hurt paint and is NOT recommended for ABS braking systems.
You may have an older instruction sheet. DOT 4 and DOT 5 are NOT compatible, never mix them! If your master cylinder specifies using DOT 4, please continue using DOT 4. PM Brake calipers are designed to work with both fluid types.
DOT 4 and DOT 5 brake fluids are not compatible and react to each other resulting in a build-up of viscous materials that can interfere with metering orifices within the brake system.
Both fluid types have advantages and disadvantages. DOT 4 is more susceptable to water contamination (minute amounts of moisture is absorbed through the seals of all brake systems), but has a higher tolerance to the heat generated by braking. DOT 5 is not compatible with ABS systems, but will tend to resist moisture contamination and is generally not harmful to paint. The choice is yours, but once you have introduced a specific type into the system, you must continue to use it.