Tom, some great info on debunking the mythical alphabet soup of lubricants. I went through this exact same exercise in 2016 with our TD MG506-1's. I went back and forth with TD and finally engaged some technical services folks at Amsoil, a product I use exclusively. I did not see any references to synthetic oils in the thread and thought I'd share our results. First, TD's spec was "any two of the three listed below" which included one motor oil, one transmission fluid and one gear oil. So, since they seemed to like motor oil in these boxes the best, I went with 15-40 Diesel oil. (synthetic of course) A couple of things happened.
First, a persistent howl in the port tranny at 1,000 RPMs dissappeared. Second, gear oil temps declined by about 30 degrees after a full day of running. Over 4,000 hours on both and they hum like sewing machines.
One common misconception is the oil will break down and go from 40w to 15w. This is not true.
As an aside, one of the docs I got from TD listed "used motor oil" as an acceptable fill. I'm thinking these things may not be that fussy, as long as you are using the right
type of lubricant, motor oil, gear oil, transmission fluid - based on your best info for that particular mixer. On the transmission tags mine say "check the manual for proper lubricant" I have the manual and it says "check the tag on the transmission for proper lubricant"