When I was a GM salesman, we traded for a Chevy Caprice. 90K miles, perfect body, paint, and interior. Only after we traded for it, we found it kept fouling the #8 spark plug. Badly. Put a new plug in the #8 hole, juggle the car around on the lot for a few days, and the plug was fouled again.
Service Dept thought the intake gasket might be leaking and installed new intake gaskets. Didn't make any difference. The car got parked on the back row of the used car lot and ignored for a few weeks. One day I asked the Sales Manager what he'd sell it to me for. He replied, "How about $100 just to get the dammed thing out of my sight". I told him he had a deal.
I ran the car out to my shop, pulled the valve covers, and found so much sludge on top of the cylinder heads that the oil drain down hole by the #8 cylinder was plugged. Oil had been piling up above the guide on the #8 intake valve. I opened the hole, cleaned up the tops of the heads and put the valve covers back on. That cured it of fouling the plug, but it still got only a couple hundred miled to a quart of oil.
I built a fresh 305 engine and figured when I had a free weekend I'd swap engines. Meanwhile, my wife started commuting to work with it, 20 miles of open highway each way. Within a month, the car stopped using oil completely. The old geezer who had owned it had just putted aroud town, and the engine was so sooted up and gummed up it took a few thousand miles of open road for the engine to clean itself out and quit using oil.
I never did swap engines. To make a long story short, we put over 50,000 miles on the car with only basic maintenance and sold it for $1200. Plus I made several hundred when I sold the engine I'd built up for it.
The Sales Manager never said another word about it, and I sure as heck didn't volunteer anything.