I had the same problem as I could not find diesel rated 10W-30. I called other tractor dealers and found that a local New Holland dealer carries two different kinds of 10W-30 and they come in sizes from a quart to a 5 gallon bucket. Now my Kubota dealer carries Kubota and Rotella 10W-30 but only in quart bottles only but I still buy the NH oil.
I believe that the NH Ambra Mastergold brand is what Messick Farm Equipment (frequent poster in the Kubota forum) recommends for Kubota tractors.