If anyone requiring ATF makes the Redline D4 ATF turn black in 8k miles, you have something going on in that transmission. A UOA would be interesting.
My recommendation is that everyone use a fluid at least for a 2nd interval prior to judging it. With all the rinsing of the previous fluid, and the pickiness of some manuals, you truly are NOT benefitting from jumping among all the available manual fluids.
I also will NOT believe that switching from ATF to RP SynchroMAX would cause ANY power or MPG loss. I've installed the RP fluid in enough transmissions and it hasn't failed in any of those units.
I also suspect that transmission fluid level might be too low or grossly too high if there was a difference among 'like' fluids. I fill my manuals when cold with cold fluid on a perfectly level surface or lift. I then purposely overfill manuals about 4-8 ounces of fluid. Unlike automatics, I haven't had any negative issues with overfilled manuals. I wouldn't overfill more then a pint on smaller units. And, some actually benefit from the overfill which quieter operation and smoother shifts.
I also read a ZF paper based on fluid viscosity and MPG. The difference in MPG dropping from 7.5 to 6cst in an automatic was 1.1%. I just refuse to believe that switching from ATF to a similar mildly thicker MTF would cause a 10% MPG change. Sorry, you need to do some investigating.
BTW, I've used and installed most of the ATFs/MTFs that I regularly recommend. I find that transmissions can be picky and sometimes you have to try other options before finding the perfect one for your driving style and transmission's tolerance.
If you are not doing the maintenance and are not watching your mechanic add your fluid, don't bother slamming the fluid. Sorry, I just find that too many times the unethical underpaid newbie techs will keep the fancy fluid for themselves while installing the shops bulk fluid. Even reputable shops aren't as reputable as you hope for. Ethics have all but disappeared in society today.
