I posted this on another forum, but figured I'd add it to this thread as it's a good thread with a lot of info...
I was a big believer in the Ford XT-M5-QS honey too, but eventually I had the 3rd gear synchros go bad while on it. When they tore it down the Ford fluid was near black and extremely thinned out. I had about the best person out there (Molakule at BitoG) recommend the GM Synthetic 12346190 fluid. His post...
"You might also want to try the GM Synthetic Manual Transmission Fluid, # 12346190. And it IS a real synthetic MT fluid with PAO's and esters and none of the pseudo-synthetic GroupIII oils.
It was specially designed for cold weather shifting."
...I asked it's kv 40 and 100 cSt stats...
"75 and 11.5 cSt, respectively."
...this is a GL-4 75w90 fully synthetic fluid btw, so it keeps you in warranty. Alternatively, Red Line has a new product out, MT-85, which lands in-between their MTL and MT-90 fluids... though it's not a 75w90 so it's not technically in warranty.
I have seen 3 of us now have synchros go out on the Ford Motorcraft XT-M5-QS honey, me being one of them, and after it sheared like that and came out so black I have no plans to put it in my brand new transmission. Sure, it does work miracles for shifting, I'm just not convinced it provides the long term extended protection I'm wanting. Molakule is probably THE most respected person on the internet about transmission and gear oils so when he recommends something, I definitely am listening. Those are some pretty high words from him about the GM Synthetic Manual Transmission Fluid, # 12346190 so I'm game. I love that lighter kv 100 cSt spec too (makes for easier shifting) and it's technically a true GL-4 75w90 fluid so that is important for getting the protection you need.
I won't lie, it's EXPENSIVE as heck at $22 a quart but for smooth easy shifting AND longterm quality protection, I'm IN. Being a true group IV and V synthetic is nice to know too. I don't like my gear oils having to use VII's.
Update to my above post, for reference so you can see this all side by side...
GM Synthetic MTF 12346190:
GL-4 75w90 ===== kv 40 cSt: 75, kv 100 cSt: 11.5, VI: , PP (c): , FP (c):
D4 ATF ======== kv 40 cSt: 34.0, kv 100 cSt: 7.5, VI: 198, PP (c): -60, FP (c): 225 (<- Automatic Transmission Fluid, you can see how thin it is)
GL-4 MTL 70w80 == kv 40 cSt: 56.2, kv 100 cSt: 10.6, VI: 183, PP (c): -50, FP (c): 232 (<- Basically a Synchromesh/max range of fluid)
GL-4 MT-85 75w85 = kv 40 cSt: 64.3, kv 100 cSt: 12.0, VI: 172, PP (c): -48, FP (c): 236 (<- Their new "in-between" stuff)
GL-4 MT-90 75w90 = kv 40 cSt: 90.0, kv 100 cSt: 15.6, VI: 185, PP (c): -45, FP (c): 240 (<- A "normal" ranged GL-4 75w90)
GL-5+ 75w90 NS == kv 40 cSt: 95.0, kv 100 cSt: 15.6, VI: 176, PP (c): -45, FP (c): 221 (<- A GL-5 fluid for transmissions)
GL-5 75w90 ===== kv 40 cSt: 115., kv 100 cSt: 16.4, VI: 155, PP (c): -45, FP (c): 227 (<- For differentials, you can see how much thicker it is)
Ford Motorcraft XT-M5-QS:
GL-4 75w90 ===== kv 40 cSt: 76.0, kv 100 cSt: 15.4, VI: 211, PP (c): -54, FP (c): 186
GL-4 75w90 ===== kv 40 cSt: 84.5, kv 100 cSt: 14.7, VI: 181, PP (c): -46, FP (c): 204