Automotive Engine vs Aircraft Engine

[Tommy]

Recently bought a brand new car and its break-in calls for a very gentle approach - avoid RPM >4000 for the first 500 miles. 

This is very much the opposite of what we do to our new engine. 

I am curious why the difference? 

[KSMooniac]

I suspect it has to do with air-cooled versus water-cooled and the associated differences in tolerances.

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[EricJ]

Much tighter tolerances in modern automotive engines than typical reciprocating aircraft engines, and different materials and assumptions about the oil chemistry.   Basically, there's not much in common between a modern automotive engine and an ancient air-cooled aircraft engine except that they're four-stroke Otto cycle and burn gasoline.