In this case whatever Kubota list them as. Should be in your service manual.
The design parameters are not in the owners manual. A part number is. Sometimes on certain filters such as fuel you will get a micron size mentioned as well.
To wit, those parameters would include, but are not limited to;
Case burst strength
Expected operating pressure range (not burst, but normal oil pressure ranges)
Expected operating temp range
Expected hours of operation between filter changes
Expected volume of product to be filtered over the expected life of the filter
Expected total maximum flow rate at maximum expected oil pump flow
Sealing ring composition (wrong type can swell or shring to much, turn soft or brittle)
Filter media micron filtration
Filter media actual square inches of surface area
Anti drain back valve present or not
Filter pressure relief valve present or not
Filter pressure relief valve if present opening and closing pressure
For example, many years ago in fleet service I did a field dissassembly on the following filters -
Motorcraft FL-1
Fram PH-8A
Wix 51515
Napa 1515
I cut the cases from the bases on all four filters. As the Napa filters are produced by Wix, little difference between them. There was a small change, less that 10% in the filter medium square inch filtration area between the Wix produced filters and the OEM Motorcraft, but suprisinly the Fram had almost 25% less filtration surface area as well as the thinest metal case.
As to the filter end cap material, The Motorcraft had a plastic end cap, the Wix twins a metal cap and the Fram a cardboard type end cap.
As to the filtration media micron size, a less than scientific experiment with a flow bench revealed all had about the same flow rate, so I would expect all have roughly the same filtering capability.
Based upon that my fleet shop went with the WIX / NAPA filters as there was a slight cost savings to be had, at no percieved loss of filter quality.
Motorcraft has since come out with the FL-1A so the game may have changed. I am no longer servicing in fleet use any equipment that uses these filters so I have no reason to inspect the current crop of these filter numbers.
Again, food for thought
David