Found this in another forum I am a member of and thought it might help. FYI, we do sell Kubota's. You'll have to make your own decision.
Kubota does \"not recognize synthetic oil\"
Just when I thought I had it all figured out.....
Meaning I believe the synthetic oil is superior to conventional oil, especially here in the north-east. But I have just completed my 50 hour fluid change on my B2710 and used Shell 15w40 Rotella in the engine and super UDT in the back. (clean screen by the way. NO metal.)
First, it was my dealer who said, "Just use good quality 15w40 diesel oil. Synthetic will void the Kubota warranty".
Next I called Kutoba Tractor Corp in Torrance, CA, Warranty Dept. They said (sounding a bit like a legal mumbo-jumbo answer), "Use what is stated in the manual. We cannot guarantee that the warranty will cover synthetic motor oil". I mentioned that the manual also says to use 10w30, but EVERY DEALER uses 15w40 (ok Bird, stop smiling, you were right and I was wrong). They had no response.
Lastly I called a mechanic worker-guy at Kubota Tractor Corp in columbus, OH. He was pretty knowledgeable but held the party line. He said, "Use a regular good quality 15w40 diesel oil. Kubota does not recognize synthetic oil". He also said that he thinks Kubota oil is 15w40 Valvoline and was in my tractor from the factory. I asked why my manual said to use 10w30 and he sorta danced around a bit. I think he said, "most commonly available oils in the US are straight 30 and 10-30 multi-vis. That may be why the Japanese company put that in the manual. (I know, we have discussed this before... but we do love oil discussions!) Also, never mind the discrepancy that Super UDT "may" be partially synthetic.
So, in the future, I believe Kubota may actually approve synthetic motor oil for use. Its better oil. But until they do, I'm going to use the old-technology stuff because, god forbid, I had a major engine failure and oil analysis indicated that I had a synthetic oil in there and my warranty was voided. This is too big an issue for the little guy to fight. I may email this discussion to Exxon-Mobil corp and Amsoil, etc. I think it would be in their interest to work with Kubota and other companies who have not yet gotten around to thinking about synthetic oils.