There was a period of time (for 10-15 years from late 90s to around 2010) when the reduction in ZDDP (for modern emission systems) caused serious problems for flat tappet engines.
However, we're well past that problem era now.
Not, we are not.
Old engines require conventional old formula high ZDDP oils and not modern synthetics.
Particularly for the OP vehicle, he needs
Mobil 1 Classic Car 10W-30 or
Valvoline VR1 10W-30 non-synthetic oils. They are high in ZDDP too.
Why do you think companies still produce those old-school oils and spend millions to supply them to customers, if the new synthetic oils cover the old specs too?
Also read what Castrol recommends for classic cars, not that is related to this thread though but it's about old and new oils:
"Thank you for contacting Castrol North America.
[email protected]
Classic cars with flat tappet cam engines represent a special case regarding engine oil lubrication. These engines have valve train configurations that require elevated levels of ZDDP (zinc dialkyl dithiophosphate) anti-wear for proper protection of the flat tappet camshaft and its lifters. Insufficient concentration of ZDDP will lead to premature wear and failure of the camshaft and lifters.
Current ILSAC GF-5 and GF-6 fuel economy grade engine oils are designed for the extended life of the catalytic converters in modern passenger cars and have industry-mandated limitations on the amount of sulfur and phosphorus within the oil.
The following Castrol oils are blended with a high zinc and phosphorus content to help prevent premature aging, wear, and metal fatigue on engines with high-tension valve springs or performance modifications that create high contact pressure and extreme internal temperatures (i.e., push-rod, flat tappet engines and performance cam applications):
•
Castrol GTX Classic 20W-50 (Conventional Oil Formula)
Features/Benefits:
-High Zinc – Typical Level 1300 ppm
-Specially formulated to provide unsurpassed wear protection for classic/collector cars & racing applications
-Compatible with gasoline and alcohol-based fuels
-Available exclusively through AutoZone and Amazon
•
Castrol EDGE 5W-50 (Full Synthetic Oil Formula)
Features/Benefits:
-High Zinc – Typical Level 1300 ppm
-Excellent cold temperature properties
-High-temperature viscosity (SAE 50) that is suitable for use in many classic car applications
Please consult the vehicle OEM or your mechanic/engine rebuilder as to the suitability of the 20W-50 and/or 5W-50 oil viscosity grade in your engine.
2. Castrol GTX Classic 20W-50 is designed to meet API SJ standards, and typically contains around 2900 ppm calcium. Castrol GTX Classic 20W-50 is formulated using exclusively Calcium-based detergents, as was typical for API SJ.
Typically, Castrol GTX Classic 20W-50 will last for a standard 6,000 mile oil drain however at a minimum, Castrol oils should be changed according to the oil and filter change intervals specified in the owner’s manual or as indicated by the oil life monitor for the type of service (normal or severe) that the vehicle is being operated under.
3. Detergent levels overall have decreased over time across the industry API SJ to the current API SQ. Typical SJ type oils contained between ~2800 ppm calcium, as do many existing products which emulate this standard. Modern engine oils contain far less detergent; typically around 1000 ppm calcium and 600 ppm magnesium.
Castrol always recommends following the guidelines of the engine manufacturer for the recommended oil (i.e., SAE grade, API rating, and manufacturer specifications) specific to your application. This information can always be found in the owner’s manual or by contacting the manufacturer directly. Before using any lubricant, ensure that the lubricant product specifications meet or exceed the lubricant specifications required by the Original Equipment Manufacturer (OEM) for that specific use.
Thank you again for contacting Castrol, we value your patronage!
Castrol Customer Relations"
