My '71 383 Chrysler was using/leaking a quart every 1500 miles or so when I got it eight years ago. An engine flush, followed by a quick change with cheap Pep Boys "synthetic", then to M1 15W-50 and new valve cover gaskets was a help.
I changed the valve stem seals a couple of years later, and oil "consumption" is at a quart per 4000. Car went from having 80,000 on it to 135,000 now.
Pulled the heads a couple of years back to de-carbon and inspect during a timing chain replacement, found no sludge in motor, just a varnish coating that a future round with Neutra or Auto-RX ought to take care of.
Car had been severely overheated after losing a water pump and having to travel a very slow 12-miles to another stop. Had been severely overheated once or twice prior to my having it.
Neutra, A-RX, Lube Control, etc, ought to be worth a try if the labor expense is as high as indicated above.