BB: from the link... an enlightening quote...
"Whatever you do, DO NOT add aftermarket zinc additives to low zinc oils. Because I did testing on this exact situation and found that adding zinc to low zinc oils, ruins an oil’s chemical properties and SIGNIFICANTLY reduces its wear protection capability. The Oil Companies always say to “never add anything” to their oil. Believe them, because they know what they are talking about. After all, they are the experts on their own products."
So sounds like look for the oil that has zinc and phosphorus already mixed in, and at levels that a 400HO was used to in March of '68 (to Bob S's point)... not sure how to accomplish that!
Great info, THX!