Mixing Redline ATF to lower fluid Friction Modifier (FM) levels.
Link to AcuraZine Optimal Percentage of Racing ATF - Testing revealed a maximum of 65% 
Link to AcuraZine   Racing ATF - Original article -  Testing went close to single digit % of FM
WARNING: The pressure switches must be replaced (if you have not done so) and replaced every few years before lowering FM.
There is a possibility that a Transmission with out of tolerance switches could cause extra clutch pack wear in transmissions with
dragging clutches, watch for any excessive deceleration during a gear change, especially 2nd to 3rd. If you notice any dragging or
harsh shifting, go back to straight Redline D4.

NOTE: Some transmissions may have very close tolerances and lower FM can increase wear when two clutch packs fight against each other, - Honda shift logic - especially in 2nd to 3rd shifts, It's important to watch for any increased buildup on the drain plug and any feeling of even slight deceleration during up shifts. If unsure, then straight D4 is the safer option.
When I changed over from straight Redline D4 to a Lower FM mix I did the following - and stayed at around 40%:

A single drain and fill of Redline Lightweight Racing fluid, approx 2.8 Liters - 3 qts. This gave me approx 60% Redline D4 and 40% Racing fluid, with a Viscosity of around 6.50 @ 100 °C. 

From then on, a single drain and fill would be comprised of:

A 50-50 mix of Racing + Lightweight will give me a Viscosity of 6.9 - Racing type F Vis is 10, Lightweight type F is Vis is 4.9.

A mix of around 1.8 Lts of D4 + 1.1 Lts of a 50-50 mix of type F above. This gives a 40% mix of Type F + 60% D4 and a Viscosity around 7.20.  D4 Vis = 7.4, Type F Mix Vis =  6.9.

There are NO harsh shifts, no harsh downshifting under grade logic, faster gear engagements that feel smooth and quick. The trans is always in the right gear with no hesitation. I preformed a 3 litre change of trans fluid after another 24,000 Km (it's not a lot, I know!) including quite a bit of mountain driving. Fluid was like new, not a single metallic "hair or fuzz" on the drain bolt magnet, only a "tiny" smear of black metallic paste. I always inspect the transmission filter and pour old fluid through a white coffee filter, both were clean, the white coffee filter was spotless with only a clean red fluid stain, not a speck of darkness or wear material. I have since gone back to straight D4 as I fing it best for this hoter climate and saves mixing and ordering different fluids. 
       Viscosity @ 100°C
        Viscosity @ 40°C 
Honda Z1
Honda DW-1
Dexron- VI
Mobile One Syn
Redline D4
Redline D6
Redline Racing type F
Redline LightWeight F
Manufacture Technical Summary.

D4 ATF is a fully-synthetic product designed with superior low-temperature properties compared to a Dexron III® fluid, and the improved shear-stability requirements which is part of the Mercon V® specifications.

D4 ATF also provides a GL-4 level of gear protection, making it a superior product for transmissions and transaxles. The balanced frictional characteristics provides smooth and consistent shifts for extended drain intervals. The superior stability compared to petroleum ATFs allows high-temperature operation without varnishing valves and clutches which leads to transmission failure.

Vis @ 100°C, cSt 7.5
Vis @ 40°C, cSt 34
Viscosity Index 198
Pour Point, °C -60
Pour Point, °F -76
Flash Point, °C 225   - Honda Z1, 186 ºC
Flash Point, °F 437   - Honda Z1,  366 ºF