The main thing with the break-in oil is additive response. There's 3 ways that ZDDP becomes active: heat, pressure, and friction. On initial startup, you have neither heat nor high pressure so you need friction. The right type of ZDDP (short-chained, secondary alkyl), that'll be more reactive at low temperatures, helps a lot as well. Common non-breakin oils, including racing oils, don't have as good of reactivity at low temperatures, mostly due to the type of ZDDP used (blended 70/30 primary/secondary alkyl), the amount of detergents used, and interference from friction modifiers and extreme pressure additives.David Redszus wrote: ↑Wed Jun 16, 2021 12:27 am Just curious.
What specifically do we expect a "break-in oil" to do that a normal motor oil does not do?
By "specifically" I mean what is the expected effect of the oil on each part of the engine: rings, valves, cams,
bearings, pumps, etc.?
Does the time period available for break-in make a difference as well?
The intent is to get an anti-wear tribofilm established as quick as possible to minimize wear during break-in as much as possible. The rings transition through all 3 lubrication regimes, experiencing mixed and boundary lubrication at TDC and BDC and ~20 degrees of rotation each direction from TDC and BDC. ZDDP is active only in mixed and boundary lubrication so getting that AW film established as quick as possible will help keep down ring wear during break-in and help with sealing. The bearings operate solely in full hydrodynamic lubrication so there's no effect there. The base oil is more important there. The valvetrain is the same with wanting that AW film on the lobes, lifters, pushrod tips, rocker tips, etc...
A common API oil like Rotella or "racing" oil like VR1 will still allow the engine to break-in, but the break-in will take longer and accrue more wear in the process.