I had same verbiage in my manual. I came to understand that the newer CJ -4 oil(s) are superior in just about every way, EXCEPT, they have a lower TBN and can't handle a lot of sulfur in the fuel. This change had to do with the new requirements for engines with particulate filters.
The ability to handle lots of sulfur in the fuel is no longer an issue in the US since high sulfur fuel can't be sold anymore.
However, Kubota sells tractors all over the world and higher sulfur fuel is still available in parts of the world. Since the CJ-4 oil can't handle as much sulfur Kubota put the limitation in the manual to cover the worst case scenario.
Originally, oil companies made separate CJ-4 oil and CI-4 because the CJ-4 was more expensive to produce and 'truckers' with pre-2007 engines didn't want to pay for the more expensive oil they didn't need because their engines didn't have particulate filters. Over time more and more engines required the CJ-4 and manufacturing costs came down and more and more oil companies moved to the CJ-4, CI-4 oil as a single product.
Not an issue when using the low sulfur fuel sold in the US.
I originally used Lucas 15-40 because they had a separate CI-4 oil. I eventually wanted to use a 10-30 which Lucas didn't sell. That is when I researched some more and decided the CJ-4, CI-4 'combo' oils would be just fine and moved to using Shell Rotella T3.
And Kubota's own oil meets both the CJ-4 and CI-4 specifications and has the lower 'TBN' number of 9 or 10 depending on the weight.
Hope this helps
