Been interested in engine lubrication since the mid '60's, but I am no engineer. Modern motor oils are superb products, and I remember when one could buy "re-refined" oil in gas bottles at gas stations. And for many older engines, I suppose that was just fine. And when if you wanted a more viscous oil, you had to buy motorcycle oil, as 40 wt was just not very available, for sure not in 10-40 ranges. And I was in Germany when Mobil 1 synthetics first appeared and when Porsche engines who used it showed up with damaged engines from failed bearings.
Modern engine oils have additive packages for things like anti-foam, detergents, corrosion & viscosity controllers, and so much more. But it is also true that just about any oil worked fine 30 years ago, and us drivers could make decisions as to viscosity and such with good results, (and some not so good results), nowadays engines are likewise extremely complex, with things like hot viscosity managing cam/valve timing. Motor oils are subject to shear, changing their viscosity, but one of the reasons for timely changes is acid contamination from blow-by and fuel and water contamination, the latter worse with shorter trips.
So it is the additive package that can be exhausted both by mileage and contamination, hence the recommendation for changes both for time and mileage, and operating conditions.
In all of our cars, they get their oil changed at no more than about half of the manufacture's schedule based on mileage.
On other car forums, there a difference in opinion when a car is not driven in winter months, changing before storage, or before resuming driving. Engines last longest if operated to temp rather than just a quick start and turn off.
As I have said before, my mantra is "...oil is cheap, engines are expensive." NAM VET
