I'm surprised that Kubota would send a statement like that - talk about behind the times. If an oil meets or exceeds the API and manufacturer standards listed in your owners manual, and you follow suggested change intervals, there's no way they can say that one brand of oil or another caused a failure.
On a related note, Delvac1 is great stuff. I race, build engines, heavily modify anything I buy, and run some high-horsepower cars. The crowd I run with and I have tried and tested just about every oil out there - Red Line, Amsoil, Royal Purple, Mobil1, and some weird exotics. Delvac1 has such a monsterous ability to hold up under adverse conditions, neutralize acids, and suspend soots that it is an absolutely fantastic oil. Believe me, you think the tractor crowd is fixated on oils? Try the folks that are trying to squeeze just a few extra horsepower out of a 500 HP engine and add 50 hours of life to it by switching from one oil to another... It's just plain ridiculous, but I've learned a lot from them.