I had my old 670 do that last year. Sometimes the check valves in the banjo fittings for the oil pump lines (where they come out of the pump) can stick, or get a little piece of debri in them (don't know how you get junk in your oil tank, but I suppose it could happen) causing them not to seal, allowing the oil tank to drain into the crankcase over the summer. I had mine actually fill my pipes as well (twin piped), until the tank was dry.
The rotary-valve shaft cavity is bathed in oil. If one of the inner crank seals leaks, it could cause this cavity to drain into one side or the other of the crankcase. The end seals of the shaft (one behind the rotary valve, the other behind the water pump), could leak as well. If BOTH crankcase halves are full of oil, I would suspect either a banjo check-valve, or the seal behind the rotary valve, as either could fill the rotary cover enough to fill each crankcase half. If only ONE side is filling with oil, there is a good chance that the crank seal on that side is bad, or the check-valve in that cylinders oil line is stuck. Process of elimination. I would start by removing the oil lines first, and clean them out thoroughly, checking on the check-valves by sucking and blowing on them. Air should only go one way. From there, it gets harder to diagnose.
I never had a problem with any of my 670's until last year. I messed around with this sled, but it kept leaking into the PTO cylinder. As it is now my loaner sled, I just said screw it, and eliminated the oil injection altogether.
