Like others have said, pay no attention to Kubota. Even the owners manuals are outdated. What the dealer sells you in the Kubota jug, is something your manual tells you not to use, LOL
I run either Shell Rotella T6 or Petro Canada Duron-E synthetics. Only thing you need to look for is you want the one that is labelled CK-4. It is backwards compatible.
Some new emissions motors (that don't apply to our emission motored tractors) call for a new oil category....called F-4. It is not backwards compatible and you don't want this for your tractor.
These will be shown in the little donut symbol on the oil container.
From the Shell Rotella web site, explaining it all:
What is PC-11? | CK-4 & FA-4 Diesel Engine Oil | Shell ROTELLA(R) | Shell ROTELLA(R)