Surprised no one has said anything about the lack of oil in the rockers on cylinder 6. Im far from a expert on this, but I think I would crank the engine over, paying close attention to the rockers on #6 cylinder and the push rods. Check for lift with your dial indicator, and possible bent pushrods.
My though is if the exhaust valve isn't opening,( or isn't opening far enough and the intake is, there isn't any place for the air to go except into the crankcase when the piston comes up on the exhaust stroke.
Like Jon, this also may be a WAG, I'm not sure.
1957 WILLYS pickup,
1973 J 4000,
1978 CJ7 DD.
1979 CJ7 360, TH400/Quadratrac.
1980 CJ5 trail Jeep.
1989 YJ the CJ to YJ.