The oil change might help.............a little. Do you have way to drain into a clean container and save if that does not work? If so, that would not be a major expense, you would want to fill with fresh/clean oil after a split, so you will be buying oil either way.
I would price the parts to replace the transmission pump and decide if the machine is worth the expense. If not sell and cut your loss.
I can capture the oil, but for the expense of it and the risk of introducing more dirt, I'd probably just dump is on a brush pile and light it up.
To re & re the shuttle transmission myself would probably be $1000, the seals are cheap, clutch disks are not too expensive and I probably wouldn't need to replace all of them. The wild card is the pump. I found a new aftermarket unit that may work for $300, there are used units out there, the worst case is a new part from Deere for $3000. The pump gears appear to be available from Deere, so assuming the housing isn't messed up, that could be an option.
End of the day, I have enough into the machine that I can't walk away from it, but not enough that if I put the parts into it, I can't recover most of the expense should I sell it.
The nice thing about this machine is that the shuttle is the most complex part of it all. If I were to get it back to 100%, I would have little doubt the rest of the machine will be reliable for as long as I own it and beyond.
I've been looking at oil charts and it would appear an AW 32 is about equal to SAE 15 where as AW 68 is SAE 20 and AW 100 is SAE 30.
I'm to the point where I think $100 in thicker oil will likely get me to where I can get the machine moving again and do some work I need to get done. Once those jobs are out of the way, I can pull the shuttle apart over the winter/spring and do the re & re at that point.