What about zinc?

Many classic car owners use "diesel oil", i.e. CJ rated, mainly because many brands have more zinc. My 1985 M-B diesel has sliding "followers" running between the overhead camshaft lobes and valve stems (no rollers) and a solid system (not hydraulic). My 1996 Plymouth 2.4L is similar, except the followers are hydraulic. Thus, many later engines seem to have the same system of parts sliding on the cam lobes. I think the main difference in diesel oil is that it has more detergents to better suspend the carbon, as they have more blow-by past the piston rings due to the 20:1 compression. You find the diesel oil is a special section at the auto parts, usually in 1 gal jugs. Rotella is one brand. I think STP is now Autozone's "store brand" (used to be "Coastal"), so not sure it is the same as in the past.