Here is some info that I copied off another site that may be of interest to some that are running stock engies.
For what its worth to the individual.
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Just posting this as an fyi. Castrol now has a high zinc content 20W-50 which should be good stuff for flatheads and others. Here's the info from their web page.

Exceptional Engine Protection for Classic Cars
Finally, a motor oil that's one for the classics...

Today's engines face different demands than those of yesteryear. Now classic car owners who are particular about what they put in their vintage vehicles can receive modern wear protection from a high–zinc formula specially engineered for classic engines.

Back in 1906 Castrol created its first automobile lubricant. Since then, our leading–edge technologies have protected generation after generation of engines. Castrol SYNTEC 20W–50 motor oil is geared to protecting, preserving and helping extend the life of your vehicle from an age gone by that still lives on.
Key Benefits *

Contains increased zinc levels for extra engine wear prevention.
Utilizes proprietary additives and base oils to reduce metal–on–metal contact of aging engine parts.
Engineered to increase wear protection for classic cars with flat tappet camshafts.
* SYNTEC 20W–50 does not meet the catalyst compatibility requirements of vehicles manufactured since 1993.

Castrol SYNTEC 20w-50 Motor Oil
Classic & Vintage Vehicles that Qualify
Pre–1949 1949 — 1961 1962 — 1973
► Historic
Restoration ► Historic
Restoration ► Historic
Restoration
► Hot Rods ► Customs ► Muscle Cars
► Street Rods ► Rat Rods ► Pony Cars
► Customs
► Pro Street
► Pro Touring

Call 1–800–462–0835 for the latest product details.

 

That's excellent info!