I'm trying to recall what website forum wherein the viscosity of the various ATF's were being discussed. I don't think it was fordf150.net, more likely britbike.com. My brain seems to want to think that engine oil is weighted, say 10-30w, while gear oil is 90w, and ATF is labelled differently. I could be way off on this, but it seems that our Walmart ATF is about a 20wt fluid. There is also a website by the 'oilguy'(?) who has researched lubricants and offers dissertations on them. I believe you will want to do some rather intense web surfing on this topic.
In the short run, perhaps if you called around to a several local shops to gather various opinions, you might gain some knowledge and pass it around back here.
Today, as the weather is warm, our early morning forays showed the gearbox shifted far easier starting out than when the temperature was about 40 degrees F cooler. It is true, the LT77 is a cold blooded beast.
Mildred Hargis