Interesting read here, albeit very long. The net of this is that he basically dismisses as myth the requirement for ZDDP (a.k.a.
zinc) that many strongly believe in for flat tappet / lifter engines, and recommends certain types of high quality 5W30
synthetic pretty much universally.
https://540ratblog.wordpress.com/
The weight / viscosity of your
oil s/b based upon the clearances in the engine, as indicated by pressures you're seeing. Standard guidance for many has been 10 PSI per 1,000 RPM at operating temperature, so 60 PSI @ 6,000 RPM. Running heavier / thicker
oil than that not only requires more power to pump, but may also result in insufficient lubrication to tight clearances - particularly when cold (yes, even with multi-viscosity oils).