Everyone will have an opinion. Mine is you should use the Kubota fluid, especially if it is a HST. Several owners have reported issues with their hydrostatic equipped machines when not using the Kubota SUDT or SUDTII fluid. Especially if you live in a cold climate.
2009 Kubota RTV 900, 2009 Kubota B26 TLB & 2010 model LS P7010
I have a Kubota B26 TLB and I used the Kubota UDT in it at first change but switched to Triple S Universal hydraulic fluid at last service. I cant see any difference in the two. As KOua said, if you are in really cold climate, you may want to use the SUDT2 full synthetic for better performance in extreme cold. Anything above 15F any brand will work just as well as the UDT.
Another issue is warranty: keep the Kubota oil in as long as you are under warranty just so they don't have any issue with using different oil. MY Kubota dealer's service manager told me this and also told me that I could use Universal oil after warranty and I would not have any problems.