>>but what would your explanation be for the greater - much greater - longevity
Better, much better engine oil additive packages. Much better control of friction and wear during the boundary and mixed lubrication regimes which exist in the start up period before hydrodynamic conditions are fully developed.
The ratio between clearance and journal diameter has to remain within sensible limits for hydrodynamic lubrication to work. The hydrodynamic pressure distributions have been reasonably well modelled mathematically since the work of Ocvirk in the 50's, and beyond a little tinkering and complication they remain valid and useful today.
However, here's the real source of the BS. People talk of improved tolerances. What that means, and what the public interpret it to mean are, of course, two different things.
When making an engineering change, you can tighten the tolerance and increase the nominal clearance at the same time.
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