ZDDP forms AW films only when there is metal to metal contact, so why do I hear that it is so important when there shouldn be metal to metal contact in the first place?
This isn’t 100% true. Yes, there has to be metal to metal contact (friction) to form anti-wear films. What most naysayers don’t explain is that there are different kinds of lubrication – boundary and hydrodynamic are just two. Boundary layer lubrication exists where there is always metal to metal contact, like with the top piston ring, cam lobe, tappets, and rockers which makes up most of the non-corrosive wear. Hydrodynamic lubrication exists in areas like bearings, where in the best case scenario, there should never be any metal to bearing contact. The thickness and speed of formation of AW wear films on metal to metal surfaces has to do greatly with ZDDP levels, which detergents are used, and amount of pressure and friction between the surfaces in contact.
