I have been visiting this topic in my head for many years.
If I could run an engine in a hypothetical lab condition eliminating outside influences such as road conditions, terrain, humidity, temperature, wind, drag, etc.
Oil y shows an average MPG better than oil x.
Would the increase of average MPG of oil y be due to the oils reduction of mechanical friction in the engine or the reduced hydrodynamic drag of the oil? Or would both contribute.
Facts please. No speculation please. References to publications would be extremely helpful.
I do this a good bit so I'll address it and the authority is the conservation of energy
By your standards where all parameters were 100% identical and equal at all ranges
If a difference occurred from the oil only- then the only mode could be a relative property that only the oil directly influences.
That would be friction from 2 body contact created the "delta"
( assuming no particulate presence to grind and change clearance or finish since this is hypothetical) That can only come from
Dimensional increase from a change in heat removal ( thermal growth)
Increase in film strength
Here's the potential paradox
If nothing else changed- it would not be possible for the other oil to achieve either because the potential energy to allow for those alterations would not exist because it was never introduced. ( if no operating parameter changed then it would not present a circumstance to take advantage of any other oil property even if it was present)
So even if the simple concept of cause/effect is employed and oil "X" achieves a performance value of "1"- oil "Y" could not exceed "1" ( it could be argued that it could achieve less than 1) unless something else changed which was not allowed due to the constraints you imposed in your model.