M1 not obviously effective on varnish on valve train. Our tow truck with GM 5.7 has 195k with 190k on synthetics, last 30 k on M1 15w50. Burns a quart evey 3.5k.

Valve cover has a horizontal baffle just below the oil filler. Varnish on that beffle is unchanged to "worse" than at 165k.

Plenty of varnish in m30 head similar M1 use. However, most any solvent, inc WD 40 seems to flush the varnish.

Maybe substituting a quart of MMytery in the crankcase is the way. Nissan 3.0 v6 with over 300k and infrequent oil service had a completely collapsed hyd lifter. GGAM what a racket. One q of MM in oil change and prob cleared up completely in 5 min .