"For what's it worth." I removed 1/2 quart of oil and added 1/2 quart of Marvel Mystery Oil to the crankcase. Also added to gas tank at the recommend amount. I've driven about 1000 miles and have had positive results with engine noise and seemingly smoother engine performance. No more lifter noise. I know the purest collector doesn't like additives but I've had really good luck with some of my older cars in years past. Especially ones that liked to smoke from possibly clogged / stuck oil control rings.