I agree with others, as the oil warmed it probably created positive pressure, though ever so slight, but enough to keep "dust" out. Now, if the entire back of the tractor was covered with 2" of dirt, that's another story. But, I'd probably run it 10 hrs or so, then change filters. That's usually fairly easy to do & it'll make you feel better about it. 

Is oil still natural colored? Or has it turned black? Usually hydraulic/rear end oil stays clean enough to keep it's natural (whatever was in the can) color.