FWIW, the past few days I had to drive the Stang in 30F-40F ambient and the 3650 felt fine using the Redline High Temp ATF. High Temp ATF has almost the same viscosity as pure MTL, but is a Mercon ATF. Redline also recommended that I use D4 ATF in my 3650.

 

I have used Redline MTL in my 89 GT with excellent results. The only time I noticed any sluggishness was on below zero days. I also pumped out the rear differential and replaced it with their recommended lube. It made a big difference on the 89GT. Shifting was smooth as silk and it quieted down. I just ordered some MTL for my 2005 GT. I am going to give it a try and will report if I have any problems as it gets cold.

 

I've been driving the Stang closer to freezing temps (32F) and the Redline High Temp ATF works flawlessly with my TR-3650. It shifts like it is 80F outside.

 

I ran a 50/50 mix of MT-90 and D4 for about a year. I was great at first, but then it started getting stiff on me, even in the summer. Once I started bogging 1st all the time, I switched to Hi-Temp ATF like metroplex. That's what I'm going to stick with from now on.