One of my really good friends back in Michigan is a Kubota salesman and also does pickups for service and repair. They see a LOT of tractors with burnt up HST transmissions from using the wrong fluid. For what it's worth he also told me ( and I verified via PDS) that Kubota fluids are made by Valvoline. So perhaps the best non OE fluid would be Valvoline. But I'd use that at your own risk, $200 in the grand scheme of a tractor investment isn't a lot.
P-3C and P-8A Maritime Weapons and Tactics Instructor, Instructor Tactical Coordinator and Mission Commander