The big killer is combustion byproducts. Since engines are a lot cleaner
burning than they once were, this is reduced somewhat. The manuals on my cars
recommend 7500mi for light service and 3000mi for severe duty. Look up the
meaning of "severe duty" and it includes pretty much everything except
non-stop interstate driving in 70F, dry, weather.

This is true! Or 'light service' could be described as 7,500 miles, all
run in one 'trip', on a dyno, mounted in a clean room. Not 'real world
usage...

Rant mode on:

Modern conventional oil's don't 'break down' in short trip usage, but
rapidly becomes more and more contaminated with water and acids...
leading to bearing deterioration, sludge issues, and hardening of seals.
Note that modern oils are still wonderful lubricants even after being
run well beyond their recommended change intervals... it's contaminates
that are the issue. (Dirt becomes more of an issue if the oil is run
long enough cause a filter bypass to open, or if you get a bum oil or
air filter. Modern oil and air filters are for the most part very good.)

If your 'average' trip is on the order of more than 20 min or so, oil
will routinely reach sufficient temperatures, and maintain them long
enough too boil off most (but not all) contaminates. These are
eliminated by the PCV. For 'long' trips, yes, 5 or 6 thousand miles
change intervals are fine.

But if your normal usage is short (or dusty) trips, contaminates rapidly
become more and more concentrated. It's here where you should do changes
every 3,000 miles, OR 90 DAYS, which ever comes first! 90 days as in
actually marking it on the calendar, and ignoring the odometer unless
it's racked up over 3k miles!

Remember, that water and acid are continuously at work, even when the
vehicle is just sitting parked. (Expensive synthetic oils contaminate
just like conventional's by the way.) 3k change intervals also minimize
dirt issues in the event you happen on a a bad oil or air filter too...
it happens on occasion.

If you don't care about the car, or trade every couple of years, run it
as long as you want! On the other hand, you can get a LOT of dependable
service out of most any engine by regularly changing oil. (Do the
coolant every year too... that's a rant for another day.)

Four oil changes a year isn't that big a deal...

Rant off.

Erik