rjh245, long story short, any premium oil will work as long as it meets Kubota's specs.
Now short story long
Unless it's changed, BX's are spec'ed for 10w30 oil for the temps that are most common throughout N. America. It also calls for a minimum if a "CF" API rateing which indicates that the oil is formulated for diesel engines. Gasoline engines require an API of "S".
So any oil that carries a minimum of a CF API rateing and has a grade of 10w30 meets Kubota's specs.
Std Mobil 1 syn carries both an S & C rating and there are posters that have used it without problems but since Mobil also makes a syn specifically for diesel engines I would recommend that over the std Mobil 1.
Theres many posters that have run 15w40 in their diesels for decades and also use it in their newer Kubota's that are spec'ed for 10w30 and they also report no problems. It allows them to stock just one oil for all their diesel machinery.
There hasn't been an oil related engine failure posted here in the Kubota sections since I've been here and folks run 30w, 5w30, 5w40, 10w30, 40w, 15w40, synthetic, syn blend & dino and will sware that whatever their using, it works great in their tractors
Theres been many the debate over using a C rated oil w/a weight of 5w30 or 5w40 instead of the 10w30 because the lower viscosity allows better oil circulation at startup, especially when it's very cold. Some have even recommended a 0w30 in the very cold climates. Makes sense to me but I rarely experience that kind of cold down here.
Me? I run the Kubota branded 10w30 dino as well as the OEM oil filter primarily because thats what spec'ed for it. I know I'm paying alittle more but I enjoy my yearly visit to the Kubota dealer to purchase it, shoot the breeze over Kubota tractoring in general and check out the new offerings.
If I were to have to order it and have it shipped I'd probably use an alternative that I could buy loically but since theres atleast 3 Kubota dealers within easy driving distance of me, I choose to use OEM maintenance materials. But thats just me, YUMV
In closing, as long as the oil (& filter) meet spec's use whatever brand you like.
P.S. You may want to check out our Kubota CUT forum, theres more folks that post there with your model (BX) tractor. Your fine here but you'll find more info on the BX's there.
2004 Kubota BX23 w/60" MMM, Thumb, AM/FM/CD.
1966 Wheel Horse 856, Gear.
1966 Wheel Horse 876, HST.
w/50" sicklebar mower, 36" rotary mower, snow blade, alternator/welder.
The USMC, providing the enemies of the USA the opportunity to die for their country since 1775.