I don't even know if they still sell it, but years ago, I used to use GUNK Engine Flush on cars that I suspected had a stuck ring or two. If that didn't improve it, I'd use a combination of Marvel Mystery Oil and type F transmission fluid in the cylinders for a 72 hour soak. Sometimes it worked, sometimes it didn't. I wouldn't waste a lot of money on maybe it's this or that. If you like the car, pull the engine and rebuild it.

 

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