I stand by the Michelin AC-10 as a heck of an off road tire for the 610. I have the 120/90/18 and its the same size as most 140/80/18 tires. It won't last long on the road, but off road it very good!
this is 250 off road miles and I'm hard hard hard on rear tires. It's a relatively soft tire, but holds up better than a MT -16 and works much better than the MT -21. Oh, and Motul 7100 10w60 full syn