A broken valve spring may be the cause of the gas leaking into the oil.Have sent comment back to person with 2002 ford explorer 4.0 v6 with problem that sounded some what the, same. There #4 cyl. had a valve spring that broke causing a real problem.My #4 cyl.is not responding when I remove the spark plug wire as it is running.Very rough running, black smoke out of tail pipe.waiting to hear back as I sent out comment.

Update from Nov 5, 2010: After a compression check on #4,#5,#6th cylinders, all pressures were holding between 126psi and 130psi. This rules out a broken intake valve which leads me to the problem being fuel injector.Another way I tested, this theory is with it running,I pulled 1 plug wire at a time,on cylinders, 2,3,5,6 there was change in the way, the engine ran [ very rough ] when I pulled plug wires 1 & 4, there was no change in engine idle. I have heard that on occasion,something can get lodged in the injector which can keep it in the open postion, and will not allow it to pulsate in a fine spray pattern.I tested this after new spark plugs.