I managed to kill my tractor way back and had the dealer send a service guy out. Turned out that I'd somehow hit some brush or something that opened the drain valve on the coolant system and the engine died of heat starvation /forums/images/graemlins/blush.gif.
(I am somewhat surprised that it still works, but that was over 500 hours ago, still going strong.)
Anyway, the service guy is quite knowledgable (having grown to know these folks over the years) and he told me that the proper mix for Kubotas, and especially the B series, need glycol based coolant (green poison stuff) mixed 50/50 with distilled water.
And yes, you should change it every two years according to Kubota's official "Workshop Manual Tractor". Darn, now I have another "to-do" item - mine's overdue /forums/images/graemlins/tongue.gif...
Get a better dealer to service your tractor.