Using a quart of Marvel Mystery oil or automatic transmission fluid does make a noticeable difference with noisy tappets (hydraulic lifters). Of course, you must drain enough oil (or do it at an oil change) to make sure you don't overfill the crankcase.
Larry
Larry Durocher
1956 Mark II C5601429
1962 RR Silver Cloud II drophead
2006 Ford F150
2010 RR Phantom drophead
2013 Bentley GTC
2015 Lincoln MKT