Oil viscosity

I have found mention that excess ZDDP can cause excess pitting and spalling on metal surfaces, includes camshafts....
How much Zinc and Phosphorus is too much,? | Passenger Car Motor Oil (PCMO) - Gasoline Cars/Pickups/Vans/SUVs | Bob Is The Oil Guy

And this documents that ZDDP does increase friction slightly.. backing up YR's statement.
Penn and ExxonMobil Researchers Address Long-standing Mysteries Behind Anti-wear Motor Oil Additive