I use the regular Kubota UDT, although I'm thinking pretty seriously about trying an "off brand". If you want to find arguments about oils, you can get an encyclopedia's worth by searching the Oils and Lubricant's thread. We've got folks that really know their stuff and they put forth arguments that I don't understand, by a long shot.
Some say hyd. fluid is hyd. fluid, others say that there is a difference. My suggestion would be to use the Kubota recommended fluid while under warranty, though, just to be sure they can't come back at you and tell you that because you used Brand X that any faliure was your fault. After that...use your best judgement.