Major plug fouling #5 cylinder

I have noticed major fouling in my #5 plug. The others are fine. I just replaced them and the old #5 was bad also. I also replaced the cap, rotor, and wires. What would cause oil fouling on this plug? Thanks!

Are sure it's oil fouling, because a wet-with-gas fouled black plug can look like oil fouling out of a freshly run engine ... did you smell of it? Or, look at it again tomorrow morning, and see if the 'shine' is gone. My suspicions are a bad, stuck injector, or a bad, shorted injector driver transistor in the DME! A dealer
'scan' by the Bosch tester seems in order. But, to answer you question directly ... one or more broken piston rings, cracked or holed piston. Nothing good can cause quickly appearing oil fouling, if it is truly oil-fouled!

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Warren Hall
1973 911S Targa

I would pull all the plugs after the engine has been heated up and do a compression test to see if #5 is different than the others. That will tell you if rings are blowing by or that a vale/valve guide problem has developed. You can check for excessive valve/valve guide clearance by trying to rock the valve stem back and forth with the valve slightly compressed. There should be minimal movement. Good luck .