I've preached on this a million times
the manuals recommend a certain fluid. IF your machine is still in warranty, and you have, say, a transmission problem, kubota (and deere) have been requesting fluid analyses. If for some reason the analysis says that a certain chemical that is present only in super udt2 or udt is not there, or in very low quantities, they can (and have) refused to repair the machine. The reasoning behind this is that when 303 or whatever fluid is used, kubota no longer has control of the lubrication, cooling, corrosion protection, seal formation, energy transfer, or cleaning properties and at that point it is not their problem. If the user had chosen super udt2, there would very likely be zero question. They'd know if the correct quantities of additives were in the fluid and at that point they're thinking "hey this guy spends a little more to take care of his stuff"....and they're much more likely to fix on their dime.
I've seen it happen. I've also seen it with fuel. Injector failure? Warranty? Fluid analysis is almost mandatory. Fail due to excessive moisture content or excessive dirt in sample? Repair is on the customer and injectors on common rail engines ain't cheap. Last I looked, about $900 each (3 or 4 of them)--plus a lot of times the rail pressure sensor is damaged, so you have to replace the rail assembly (only way to get the sensor....IIRC it's about $2100). All because the owner saved $20 on some aftermarket fuel filter that is of unknown quality. "But it came from the same factory"....might have, but mind you that factories often make a subpar product specifically made for box stores, parts stores, discount stores AND they also make a product (which is often better quality as specified) for the OEM's. Same factory.
I went through this with Yamaha. A company makes Yamaha filters for them so everybody thinks they're the same, when in fact, they're not. One's made of inferior materials and it goes to another place to be labeled with fancy colors and stuff, then sent out to discount places. Same for fluids, also went through that with Yamaha. Same place mixes up different chemicals, one specified by OEM (Yamaha in this case) and it bottled and labeled as Yamalube, and right out of the same door, another set of chemicals, which in this case didn't really play well with wet clutches, bottled and labeled with a pretty color and lots of marketing gimmicks that make you want to buy it, and then some dude gets it and sends it out for analysis and puts on his website that "it's fine" and cheaper than Yamalube. You never hear the whole story, though. The question is, was this dude a paid advertiser? Or is he brand loyal? Or????
Stick with OEM, you'll be fine every time.