I run all my older engines on Chevron Delo 400 SAE 15W-40. I formulated that oil for Chevron in the 1980's and 90's and it works very well at keeping dirt suspended as it is designed to keep soot suspended in a diesel engine for 15,000 miles oil drain interval.
HDDEO (Heavy Duty Diesel Engine Oils) have more ZnDTP than Passenger Car Motor Oils (PCMO) as they don't need to meet the EPA 150,000 mile emissions requirements. The EPA requirements are why the modern PCMO oils have less ZnDTP.
I ran Delo 400 in my 426 Hemi Belvedere, '67 383 FB FS 'cuda, and numberous 340's. Never had a camshaft issue.
What I kike about 15W-40 is that this viscosity grade has great pumpability at startup, excellent oil pressure without over pressure, and great ring and liner wear protection. Rings wear at top ring reversal due to lack of squeeze film lubrication. That is the ability to prevent rings from touching the cylinder liner when the piston stops moving at TDC (BDC is a low pressure event so no real ring wear at that point.).
Older engines are designed to use SAE 10W-30 for the most part and work well on 15W-40, but heavier oils can have high oil pressure and cause drag in the engine, especially when cold. Light oils like 5W-20 don't work well with older ring/liner/cross hatch combinations normally seen in rebuilt engines. To use light oils and narrow rings, the whole system has to be matched.
