I'll repeat what the others said. there the same thing only difference is that the offroad is dyed red to be used in non taxible senarios, like off the road applications like marine applications, generators, dirtworks equiptment and ag applications. So there is no tax on it.
That being said, in the past and still in areas where i live offroad deisel is Low sulpher deisel(150ppm) and on road is Ultra low sulfer deisel (15 ppm). Both are ok in your tractor as it was designed to run on high sulfer deisel. The off road is prolly better as it has more lubrication qualites and more energy per gallon. But no need to worry or ask and frive all over to get it the ULSD is fine.