What is the best fluid to use in the TRANSAXLE (5 speed)

[Sid_RallyX_82]

Original fill (yes, I was working on these when they were new) was a low-viscosity oil with anti-foaming and surficant agents, undyed. Ford's service info called for replacement with Mercon ATF. That was the closest to original fill that was in Ford's inventory. When Mercon V became available in the early 90's, it was excluded from the approved fluids for manual transmissions and transaxles. Merc V synthetic has too low a coefficient of friction for synchronizers. Synchro blocking rings are essentially brakes grasping the speed gear, and matching its speed to the shaft on which it lives. Merc V didn't allow enough grab, resulting in grinding during shifting. The other approved fluid was 10W-30 S class engine oil. However, I've found this to suffer from aeration at extended elevated speeds. After the discontinuation of Mercon, and prior to the availability of the Motorcraft XT-M5 oil, I used a 50/50 mix Merc V and 10W-30. Fluid exhibited little to no foaming, and lower shift effort than straight Mercon. I found the XT-M5 fluid to be far superior in shift feel, smoothness, and effort to prior fluids. A cursory temperature measurement with an IR gun shows slightly lower temps after sustained highway speeds as well. I have no experience with aftermarket fluids in these units.

Would there be any reason to do a flush when switching from Mercon V to XT-M5? Or just drain and fill is fine?