When you had the head gaskets done should have had valve job done with new seals, check guides imo

 

Sorry, I would beg to disagree with you. Oil has different additives and packages as per the spec sheets per manufacturers. Oils are designed by engineers for applications and are quite specific. The oil used in aviation is quite specific... I would not board a plane whereby the mechanic told me that he didn't believe that they needed oil with a package that met manufacturer recommendations and decided to go with oil from Walmart off the shelf... because oil is oil... yeah... no...

Tolerances and clearances in engines are specific and specific for certain oils. Would you take a brand new car today that has a manufcaturer spec of lets say 0w30 and say, you know, who needs that, I can get away with 20w50 because it is the cheapest oil and I don't think it requires any of those "fancy" packages?...

All oils are not created the same, as the packages/additives are specific to applications and pressures.

The ZDDP thing is not over blown. It is a 1960's buick engine... back in those days oil was not as refined or clean as it is now, and ZDDP was common in almost all of the oils back then as a lubrication package. Since, engines have changed, and the LR and Corvette engines are the only ones that I am aware of that are flat tappet designs.

ZDDP was removed due to the inclusion of catalytic converters as the ZDDP essentially killed them.