Shell Rotella Full Synthetic 5W-40 and I'll second the owners manual states the wrong capacity. If the tractor is new to you, be aware there are two fuel filters. One is easy to get at and the other takes a bit more work and is located underneath. If you don't know the service history, I'd consider changing the front axle lube also. Use conventional gear lube.
FYI and not related to the title... My Dad has the same tractor and recently the electric fuel pump failed. The Kubota replacement is rather expensive and we ended up using an aftermarket automotive electric fuel pump that was a direct bolt in for a 1/3 of the price.