I use Mobil Delvac 1300 Super in 15W-40, in my 2001 Kubota BX-2000. Change oil and filter (Wix or Kubota) every 100 hours and I have beat that Kubota like a government mule for the past 17 years and still runs fine. Again, heavy duty engine oil is a must with these tractors and their industrial diesel engines. If doubts remain you call Kubota and say, "My tractor is spec'ed for 10W-30 or 15W-40 engine oil, and requires CJ-4 or whatever, but can I use 0W-40 instead that is formulated for German automobiles and not rated as a heavy duty engine oil, and will the Kubota tractor warranty cover my use of it?" Because if Kubota would not pay for warranty engine repairs on their dime even when a tractor was new, that is a pretty good bet you are not supposed to use it without assuming some risk.
They are your tractors though. I understand these diesel engines are also used on stationary generators, a completely different application -- but a stationary engine is not running under a small enclosed hood and moving around over fields and ruts in a hot, dusty environment that is kicking up dust and mud and bits of brush, like a tractor does daily, and possibly mowing grass that blocks the radiator screen with clippings, etc.
