Ken, if you can take some pics and post them along with your notes and comments - maybe different threads for the hydro and the rear axle - it will be a big help to other ride-on owners, as you said.
Auto trans fluid has a lower "base stock" viscosity than SAE20, but it has lots of viscosity improver to make up for it. I strongly recommend that you use what the hydro manufacturer specifies - those things are awfully expensive and fairly delicate. Make sure you clean around the filler before you take off the lid, and generally keep things seriously clean. Is there a procedure for changing the hydro trans fluid? It probably needs it.
You'll need to take a careful look at the drive belt of course, in case it is worn.
