the black coating on the lifters is sacrificial .
the base of the lobe definitely drags on the lifter excessively
PDs should only use thick oils in the 50s range
Here is my feelings on the PD issue. After reviewing the scientific literature you need the inlet temperature at the lobe follower interface( EHL oil film) and the
base oil viscosity at that temperature designated by the 5w, to determine the oil film thickness in the EHL( elesto-hydro lubrication).
The 40 or 50 viscosity designation is polymer generated and temporarily shears in the EHL and therefore is of no consequence in determining oil thickness in the EHL.
In fact this is why I want people to look at the vintage motor oil mono grades and note the flash temperatures. The 20w20 mono grade also does not have any polymers and will equal a 15w40 synthetic in the 105 C range. The 20w-20 may equal the 5w-40 synthetic in the 110 to 115C range.
Edit I now need people to read this before we continue
http://www.synmaxperformancelubricants.com/PDFs/SynMax_UNV_Automotive_ Base_Oil_Presentation.pdf
Going back to my last post. I was preparing one to understand that a solvent refined group 1 single grade oil non detergent motor oil with a viscosity of 13 cst is the ideal break in oil with ZDDPlus added assuming a follower inlet temperature of 105-110 C.
I want to make three things clear
1) you can formulate a break in oil specifically for the PD based on mixing mono grade motor oils.
2) The black coating can cause problems when it starts to wear through if it fractures( very small fragments that scores the underlying substrate) due to stress loading by the EHL oil film load or actual asperity contact in boundary or mixed lubrication. I am suggesting you need an ester/pao mix oil to prevent this happening.
3) If the black coating is worn away and a smooth follower substrate( carbon steel) is exposed then you can run any of the oils that are recommended because the additive packages are designed for steel on steel.
