"Your machine - your wallet" sums things up nicely, but here's why staying with the factory recommended SUDT and/or SUDT2 always made sense to me.
1. The hydraulics are the heart of these tractors; if the plumbing gets boogered up, nothing works properly.
2. Having spent what for me was major money on these tractors, I couldn't see going with bargain-priced hydraulic juice just to save a (comparatively), few bucks.
3. Of course you can't believe everything you read on the internet, but there have been a fair number of folks who said their HST made more noise when operated with a lower grade of hyd fluid. Even Kub UDT was reportedly not as good as SUDT or SUDT2.
There are some good non-OEM products available that some folks favor. If it's indeed equal or better than the factory stuff, could be worth a try.