After using the AMSOIL fluid all year and just below freezing, your post confuses me a bit. My gear shifts always go in very smoothly. The stiffness the OE fluid had in reverse and downshifting into some gears was either reduced or eliminated. Once warmed, the shifting was perfectly consistent and perfectly smooth into each gear. Full disclosure, I don't drive in winter conditions with this car if I can help it. I have a winter vehicle, so sub-freezing shifting performance was not on my list of priorities.
Ford's XT-M5-QS is significantly thinner in viscosity with a cSt in the low 11.x range (AMSOIL's 75W-90 GL-4 is at 13.9 cSt at 100C by comparison). It's basically a synchromesh fluid like AMSOIL synchromesh, GM synchromesh, and Pennzoil synchromesh, all of would provide similar results. The problem with these fluids in my experience is that while they shift a bit smoother when very cold, they tend to get inconsistent and occasionally notchy when hot as they thin out. I chose the 75W-90 GL-4 to prevent the hot temperature inconsistency I've noticed switching between the two in another vehicle and comparing to the OE fluid in the 124 Spider.
For year-round shifting quality, a synchromesh fluid like Ford's option may be better, but it should be acknowledged that there is always a trade-off on manual transmission fluids. All oils thicken when cooled and thin when heated. The transmission is comfortable operating in a certain viscosity range, and once you get out of that range either by a thicker fluid being run too cold or a thinner fluid being run too hot, it starts to behave inconsistently.
Have you run this fluid hard at the 124 Spider's power levels in summer heat? How cold is it there now?
Last edited by XtremeRevolution; 01-03-2018 at 04:44 PM.