I can tell you that I’ve seen a number of vehicles come in on the hook with engine problems due to extended oil changes. Some were moderate in nature and others were severe enough that the engine was total scrap metal. A number of these engines did not even have 30k miles on them.
One VW had a sludged engine at 10k miles although at this point it was not as bad as some. The cam lobes were galded so it was still a pricy fix.
A dealer I worked for bought a number (approx. 30-40) Chevrolet fleet cars and every car had approx. 35k miles on them. Every single one of them was sludged up and slightly smoking.
What was the common denominator on all of them? Extended oil change periods.
On a brighter note, things like this lead to more work for the shops and as the old Fram TV commercial used to state, you can pay me now or pay me later.