Marvel Mystery Oil has been around longer than I have been playing with cars, which is a long time. It's an oil additive with high detergent properties. If the lifters are sticking, it can help clean them out. The home-brew flush described by Old_Man will also do it. I have used that, too. Note that with his method you don't run the engine for long with that witches' brew in there, and you don't drive it hard because it really thins down the oil and you don't want to score the bearings. Put the stuff in, warm it up to operating temperature in the driveway, then dump it and do a full oil and filter change.
Follow the instructions and you can drive it with Marvel Mystery Oil in the crankcase.