I see a small flaw here, which is likely easily fixable. No two cars are fully identical: usage, patterns, weather, etc... When we notice the sound difference - it's always the same vehicle, just at different times and old/new oil in the crankcase.
I also mentioned (either here or on other similar threads) that I can notice by valvetrain chatter which engine is low on oil. That happens on the vehicles of friends and family that I maintain. Since I'm the one servicing them - I know how quiet those engines are with fresh oil at full capacity. So when my God given hearing sensors go off - it often ends up being that the vehicle in question is closer to the end of its OCI, or is currently low on oil.
I guess I should also note that I come from a musical family, we all sing in choir or solo or groups, and all have hearing that is trained to be extremely sensitive to changes in tone, or changes to a single instrument in the orchestra, and so on. So when the engine orchestra is slightly off note - I spot it quick, but to an average DIYer it may not be as obvious.
