4 1/2 or 5 quarts of oil with 1/2 quart of ZZDP?

Correct S'Cuder,
Current engine oils are designed to last perhaps up to 10,000 miles between drains and therefore are highly over-formulated for engines like the older ones we are dealing with. The big issue today is timing chain wear as so many designs with OHC run very long timing chains that can stretch even a little and throw off timing. So there is a specific engine test required now under API specifications to show that the oil can protect against that wear.
Currently, the testing protocol for certifying a PCMO package is probably $2 million each product that is licensed under API. GM has its own certification under DEXLOS that is a step above API certification. Any oil with Dexlos label will be essentially a synthetic or the performance equivalent thereof. No need to spend any more money on a name brand product. If it has a Dexlos approval, it is overkill for all modern engines. Just don't use SAE 5W or 0W-20/30 oils in older slider follower engines.