That turns out to be a more complicated question for a new tractor than one would expect. Just before my B2650 was delivered I was at the dealer and asked for two quarts of whatever oil was in it, and two quarts of SUDT -2. When I got home I discovered that the 2 qts were CJ-4, which is for engines with a dpf filter, eg the 2650's bigger brother the b3350. The engines themselves are basically the same design on the two tractors, and so I could see Kubota for the sake of convenience might put the same oil in all of them. The CJ-4 bottle says that it is backwards compatible with the oils listed in the manual classes CF, CF-4, CG-4, CH-4, or CI-4. For me CJ-4 would be convenient because my other tractor requires it.
Kubota recommends against mixing oil types, so if you are ADDING oil to your new tractor, check with your dealer to see if they know what is in yours and get a quart of the same, and then after that if you change it at 50 hrs you can use any oil of the same class and the appropriate weight for your climate.