Originally Posted By: Shannow
Originally Posted By: dnewton3
And then we'd have to factor in the effects of the TCB (tribochemical barrier) that is promoted via heat oxidation, and actually HELPS reduce friction, as proven in SAE 2007-01-4133. Perhaps the reduction of friction off-sets the slightly thicker vis pumping energy resistance, and therefore the end result is a neutral effect?
You are talking a (misapplied) boundary effect paper versus fundamental physical rules of shear rate, viscosity and dimension. Viscosity effects remove the contact area from the boundary lubrication regime, so one would expect that the boundary friction reduction from used oil has a lessening effect as the viscosity increases...they don't cancel.
To the OP, used or new, the difference is probably comparable (or less) than changing brands...minuscule.
The OP asked if older oil runs warmer; to any practical degree, no. To a theoretical degree, possibly, but you'd have to consider ALL effects of ALL variables, and that would include the offset of the TCB, which is a proven phenomenon.
I will note that the study I reference specifically speaks to the wear reduction being directly attributed to the effect of the TCB and not the vis shift. Page 6, last paragraph, indicates that a 20% shift is vis did nothing to change the wear rate contrasting the two oils noted. Specifically, it was the chemical boundary layer. Similarly, the reduction of friction was noted as the TCB increased, and then stabilized, and was not attributed to the vis change; pages 6-8.
But the discussion is moot, IMO, because the shift in Vis and the effects of TCB are difficult to distinguish in terms of everyday thermal effect. It would be near impossible to prove that "old oil runs warmer" because the delta-T we'd be looking for would be obscured by other variables too difficult to control.
It makes for great fodder, as I said before. But it cannot be reasonably proven and it too difficult to separate the minutia of contributors. It's just idle BITOG banter and nothing more.