Well, Red Line has 3 GL-4 fluids. Their MT-90 which is a 75w90 and the MTL (basically a Synchromesh/max replacement in terms of kinematic viscosity at 40 & 100C cSt ie: 70w80) and also they now have an interesting fluid called MT-85 which is a GL-4 75w85. Technically this would be out of spec too, but to me it's the most interesting as it's an in-between Synchromesh/max and 75w90. This is the fluid, if warranty was no concern, that I would use from them as it likely has even better shift feel than the Ford Honey but also better protection than the Synchromesh/max's. Including their own. I may even give it a go anyhow as I doubt any issues would arise using it (imo).
They also offer a GL-5 75w90 for transmissions they call 75w90 NS which carries the newer GL-5 spec for trannies in the right kv. This is a tad bit thicker at kv40C cSt than their GL-4 75w90. This is not to be confused with their GL-5 75w90 gear oil (for differentials) which doesn't carry the "NS" nomenclature. However, GL-5 tranny oil's are a concern for the "yellow metals" having an issue with corrosion from the extreme pressure additives. Though there is some debate now, from some very smart people (see bitog) btw, where they believe this is no longer an issue with the modern GL-5 rated oil's for transmissions. I could link you to a few threads where this discussion is played out, but if a warranty is a concern, then don't even consider it. Really there's not a huge issue to move to GL-5 from GL-4 imo anyways so it's probably not worth worrying about. Though I do agree they need to get off their collective butt's and re-address this issue so newer fluids can finally move forward. I wouldn't myself want to try the GL-5 75w90 NS anyhow as it's a tad thicker at kv40 C cSt, which would make it shift worse in the cold than even their GL-4 75w90.
So the safe bet still ends up back as the Ford Motorcraft XT-M5-QS as it's still a GL-4 75w90 required for warranty but as thin as possible at kv40 and 100 while still retaining this specification. Though as I said earlier, for me, I'm going to try the new Red Line GL-4 75w85 fluid as I'm willing to take that chance. I know it's going to be fantastic shift quality.
For reference so you can see this all side by side...
For transmissions:
D4 ATF ======== kv40 @ 34.0, kv100 @ 7.5 (<--- Automatic Transmission Fluid, you can see how thin it is)
GL-4 MTL 70w80 == kv40 @ 56.2, kv100 @ 10.6 (<--- Basically a Synchromesh/max range of fluid)
GL-4 MT-85 75w85 = kv40 @ 64.3, kv100 @ 12.0 (<--- A new "tweener" fluid, but even lighter than the Honey! not in spec tho)
GL-4 MT-90 75w90 = kv40 @ 90.0, kv100 @ 15.6 (<--- A "normal" ranged GL-4 75w90)
For differentials:
GL-5 75w90 ===== kv40 @ 115, kv100 @ 16.4 (<--- You can see how much thicker it is)
And for more reference, Ford Motorcraft XT-M5-QS = kv40 @ 76.0, kv100 @ 15.4 (as you can see, this lies right between the Synchromesh and 75w90 GL-4 at kv40 C cSt, but actually is still within the proper range of a 75w90 GL-4 at kv100 C cSt, this is why we all love this fluid! The greatness of shifting in a lighter viscosity fluid at all temps but still technically in the proper spec to maintain warranty claims. I'm curious myself as to what the majority of their basestocks are, as the must be of extremely high quality or a lot of viscosity index improver's.
This is why people who end up trying a Synchromesh level of viscosity fluid rave about how great it shifts. But they're not realizing the problem with this. Protection. For this matter, one could just go ahead and use an ATF fluids for even better shifting properties, but they don't obviously because they realize they would be out of spec and would like cause wear issues eventually. Especially in the heat! For these people doing this, I would recommend they move to something like the MT-85 or AMSOIL's new equivalent offering in the same range instead of staying with a Synchromesh/Synchromax. At the very least, in the heat of summer most especially if racing it in the summer!
So if you are tracking your car a lot, you may want a more conventional GL-4 75w90 for the protection of the thicker viscosity would probably handle the high temps a lot better. But if you just daily drive and it's in the cold you may want something thinner. This is why I was thinking of trying the Red Line MT-85 or the new AMSOIL equivalent (they have one too in this range) for a winter run and then going to their MT-90 for a hot summer run when tracking/racing. Or else if I want an all around fluid for a year round solution in a climate that gets really hot and really cold like the Midwest, the Ford Motorcraft XT-M5-QS "Honey" would be the pick. Which is why I say it's the best all around choice for us. A set it and forget it fluid.
Having said that, since I've had no issues. Shift feel has improved. Although I'm not apposed to changing it up again if something better is presented. I didn't know when the last time this car had ever received a trans fluid change. So knowing it was punished after almost 100 degrees ambient temperature and god knows internal temp for around 10 hours I was looking forward to changing it to anything after getting back from vacation. I missed 3rd a few times cruising around Myrtle, too... 
