I'm with Patrick. Heavy Duty Engine Oil (rated CI-4/SL) is the way to go for protecting flat tappet valve trains. The Corvair Oil article recommends this as well. It also suggests that phosphorus (the anti-wear component of ZDDP) levels higher than 1400 ppm lead to long term wear and engine deposits. Phosphorus levels over 2000 ppm lead to camshaft spalling.

I would be very careful about adding more ZDDP to engine oil. It's better to use an oil that already has the right additives in it. HDEOs have better detergent additives than racing oils for street engines and can go longer between oil changes (or keep engines cleaner with short oil changes). Even the latest emissions HDEOs (CJ-4/SM) still contain 1000-1200 ppm of phosphorus, which is more than the previous Starburst oils contained (ILSAC GF-3 - 1000 ppm max of P). So yes, 15W-40 Chevron Delo 400 (CI-4+/SL) has been reformulated into a 15W-40 Chevron Delo 400 LE (CJ-4/SM) but it still contains 1200 ppm of phosphorus.

Incidentally, the latest API SM Starburst oils still contain 600-800 ppm of phosphorus (ZDDP) and a GM study found that (OEM-style?) flat tappet valve trains only require 500 ppm of phosphorus.

Shell Rotella T CI-4

Chevron Delo 400 LE 10W-30

Heavy Duty Engine Oil List