i have raised the zinc argument before and you cannot find anybody who isn't convinced it is necessary for flat tappet cams.
It has become a tremendous money maker.
Sure. Just as Sulfur isnt ''necessary" in diesel fuel for a diesel engine to run but it sure helps.
im sure the older engines would run just fine on a oil with a lower zinc and phosphorous ppm but i believe it helps in longevity concerning wear.
Just as is when the gas had lead in it. now its all lead free, sure it runs off of it with no issues but there were benefits to the lead.
but along with everything the epa changes it constantly and thats what really determines whats available to use