There is a lot of information about engine oil for flat tappet cams and I think one of the best articles is this one:
I used it plus some other sources to write this article:
http://www.raso.name/index.php/cars/22- ... engine-oil
For engines with flat tappet camshafts, I use a Heavy Duty Engine Oil (HDEO - has both diesel C and gasoline S ratings). Engines need oil FLOW rather than oil PRESSURE and pressure is an indication of flow, viscosity, bearing condition, and temperature. Engine oil pumps are positive displacement pumps, which means their flow is proportional to RPM and they can generate extremely high pressures because oil is incompressible. If oil pressure is too high because the oil is too thick, the pressure relief valve spills oil back to the sump instead of the bearings where it would be more useful. Because of its internal fluid friction, heavier oils run hotter than thinner oils in bearings, under similar conditions. It's usually pretty safe to go with your engine manufacturer's viscosity recommendations. Excessively thick oils can also starve an engine from lubrication in temperatures.
The antiwear additive in oil is ZDDP and it is the phosphorus component that does the protecting. Energy saving Starburst oils are limited to 600-800 ppm of phosphorus to protect catalytic converters. HDEOs with a CJ-4 rating are limited to 1200 ppm of phosphorus (P) and generally 1000-1200 ppm is good for older OEM-style flat tappet engines. For high lift cams with high valve spring pressures can up use up to around 1400 ppm of P. Excessive amounts of P can lead to camshaft spalling. HDEO with a CI-4 and earlier aren't limited to 1200 ppm so you would generally find high levels of ZDDP in CI-4 15W-40 grades.
Valvoline has racing oils with higher levels of ZDDP but I don't recommend them because racing oils would have lower levels of detergent/dispersant additives necessary for street-driven engines.
I've been using Rotella T 10W-30 in my slant six lately. Shell doesn't have very good product data sheets but BITOG has virgin oil analysis for this oil that shows 893 ppm in 2007.
http://www.bobistheoilguy.com/forums/ub ... ber=812467
http://rotella.shell.com/rotella-produc ... ction.html
Chevron Delo 400 15W-40 has 1390 PPM of P.
https://cglapps.chevron.com/msdspds/PDS ... Format=PDF
Petro Canada's Duron oils are rated CI-4 but their data sheets don't list the phosphorus content. PC's tech department gave me the following information about the phosphorus content of their oils. If you want to run the maximum phosphorus and your engine doesn't burn/leak oil, Duron Synthetic 5W-40 is a good choice. Otherwise, I would run Duron-E 10W-30 which is priced similarly to Rotella T so I'll be switching to it once I run out of Rotella.
- Duron Synthetic 0W-30 – 1244 ppm
- Duron Synthetic 5W-40 – 1435 ppm
- Duron XL Synthetic Blend 0W-30 – 1160 ppm
- Duron XL Synthetic Blend 10W-40 – 1258 ppm
- Duron XL Synthetic Blend 15W-40 – 1284 ppm
- Duron 15W-40 -1254 ppm
- Duron-E 10W-30 - 1200 ppm