If your in cold country the Kubota SUDT is recommended and UDT for everything else. There are other brands out there that will work just fine like Mystik.
I use UDT in mine - the local tractor place sells it - but they also sell "Universal" Tractor Fluid. This is the equivalent they tell me. On the drum it had viscosity 5W20. Which seems about right. The price differential was minimal, so I bought a drum of Kubota UDT. 80 weight oil gets used in the front axle and steering box. The gear box oil also does the hydraulics, so I can't see 80 weight oil being the right answer...