OOOOOPPPPPSSSSS! I should have let this one go but... When the oil is cold it is too thick to flow thru the tiny oil pump in you engines. .
I have commented this many times on the forum. This was very true 40 or 50 years ago, especially with mineral mono-grade oils of lower quality.
Nowadays, good semi-synthetic 10W-40 oil has the same lubrication (flowing) characteristics at -30 C and +130 C, so warm vs. cold oil is not an issue anymore.
The biggest problem is the time it takes for the oil to reach all parts of the system after initial cranking, and this is why manufacturers have started adding "stickiness" additives, usually magnesium based,so enough oil "clings" to various moving parts of the engine until fresh oil flushes everything upon start. I know that for car engines, it can take 10-12 seconds for the sump oil to flow fully through some large engines during cold start, and it is probably just a few seconds for small scooter engines.
I am aware you said that this topic is valid for those "anally retentive", so I am just broadening the perspective, and my opinion is that oil flow during cold start with good quality oil of modern oil companies is not a serious topic. It surely was in 50's and 60's.
It is true that the engine part dimensions change with change of temperature. Temperature expansion coefficient of aluminum is, for example, roughly double the TEC of cast iron. Considering that engines operate in temperatures between -30 and +120 C (extremes), parts are not made to be perfect fits, but certain tolerance is calculated and oil, gaskets, O rings etc. are used to fill the gaps and provide compression. It would be, in fact, very difficult to comment if the engine runs better in terms of parts fitting "better" when cold or warm. This may sound like a paradox, but engine is a collection of connected bits that are worn differently, therefore gaps are different in different parts, and these gaps increase as the engine ages. It is true that thermal expansion (warm vs. cold engine) might make up for some of that wear, but new engine is not fitted to be "tight" either. Really, the system is too stochastic to draw any definitive conclusions about this.
However, undoubtedly, engines that reach working temperature of 90 C run the best and wear the least and it is only logical to reach that temperature using the engine in moderate rev range and driving, and not wearing it while it is stationary, and then wear the engine by using it (driving it).
