The old adage "You learn something New every day" certainly continues to ring true throughout my life.
After doing more research on this, I have found plenty of eg's of large mining and other commercial heavy diesel operations that use 15/40 diesel motor oil in place of hydraulic fluid. Although, most of these were simply seeking a more cost effective/ convenient method of only having to stock One type of oil. Never the less, if there would have been a significant cost to pay in greater maintenance or break down, I'm sure this is where it would have been seen.
Growing up and being mechanically inclined from a pretty young age, I always respected that the correct fluid needs to be used in the correct application. Understanding that hydraulic fluids, trans fluids etc, could have some interchanging because the applications are all Hydraulic. Brake fluid for eg. being at the Top of the list and not being able to be replaced by anything else. (although I do remember some telling me you could get away with trans fluid in an emergency, but not sure I ever bought that)
I Never heard of motor oil being referred to as a hydraulic fluid and have always separated it from the possibility of being used as such.
After looking further into this though, it would seem that I have been ignorant of the fact that All fluids to some extent are considered hydraulic, simply from there ability to be compressed without breaking down and loosing all volume.
So my over all views have shifted about this, which is also the reason I posted this up originally. I have found that by posting something and seeking scrutiny from others, and as long as you can remain open minded and unoffended, you will most likely come to a greater understanding of what you seek. Whether you are right, wrong or indifferent, is really neither here nor there, as long as the Truth is able to prevail, or at least your ability to understand what part of it you can at the time.
It makes sense to me now, that the reason for running motor oil in these trannies could simply come down to Heat. If the #1 failing grade is from to much heat, breaking down the hydraulic fluid, then motor oil shows a clear advantage, as it is meant for a much higher temperature resistance than hydraulic fluid. Also, and I'm not sure if this is still true today, but hydraulic fluid was not available in a 50 weight. So again, if too low viscosity is the issue, then motor oil covers that better as well.
Why JD seems all over the map on what they recommend, across the years and models still seems strange though. I would have thought it would have more to do with regions, like with some vehicle manufacturers back in the day, running lighter viscosity fluids in manual trannies in Canada because of the colder winters.
Make no mistake, IF I lived in Texas, I would be telling you all that my plan is to run 20/50 full synthetic motor oil, and it would have been that even before I knew what I know now.
However, I live in Canada where we rarely see temps in summer reach 90 deg. Usually mid 70's or cooler. So I'm still inclined to consider the Hy tran as a viable option.