My W211 has #5 cylinder oil soaked plug and looking nasty from the borescope pics. Blowing grey smoke out of the exhaust and using a quart every 500 miles.
2009 4matic wagon with 142k, and body in good shape.
I'm not sure what to do as "smokey" is now missing on #5 and cel won't clear. Any thoughts out there? Does this sound like a recall item?

compression test? leak-down test?

Which engine are you talking about?

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CENSORED due to not family friendly words

Just swap a good low mileage engine a weekend job

Are you sure your plug is oil fouled? Grey smoke would be more a fuel issue. Running way too rich. You may want someone with an SDS to go into the engine fuel management and see what type of fuel it's set up for. I've seen these switch on their own to E85 causing a extremely rich condition, flooding one or more cylinders that result in low or no compression. I think that there may even be a software update for this. If that's the case, reset the values for the fuel system, change the plugs and run until it clears. I may be way off base, but like I said, I've seen this a few times when the customer was ready to throw the car away when it was an easy fix. Although, everything that I just mentioned wouldn't explain the high oil consumption. So, I could be wrong.