I think the trick is to understand what the product is, rather than what the marketing claims it does.
My best guess from reading a few threads here and elsewhere is:
- SUDT2 = Premium UDT = 75% synthetic. Goes better at low temps and high temps than mineral oils. Since it's only 75% synthetic, you can mix it with UDT (and it says that on the pack). Depending on how much UDT vs SUDT2 you get, you may be only 50% synthetic or whatever, which doesn't hurt anything but gives you a less premium blend
- UDT = largely (entirely?) mineral oil based hydraulic fluid. It's fine, and it's certified for all Kubota tractors. For most residential use you'll never notice the difference. If you live in a very cold or very hot place it may matter. If you live in NZ like I do, they don't even sell SUDT2 at my dealer (NZ has a very moderate climate, never really too hot, never really below freezing). But NZers are notoriously tight with money (or so Kubota seem to think), so there's lots of premium Kubota products we don't get. Our tractors still get the same warranty though, which suggests that SUDT2 isn't critical.
Of course, this is a religious debate not always fact based, and I'll never criticise someone who spends a bit more for a bit better product. If SUDT2 was available here I'd probably buy it, even though I know I don't really need it.