I change the oil in my home mower before each use, or every two
cuttings at the most. On US made single cylinder simple air cooled
engines the oil is the lifeblood and dirty oil will greatly shorten
engine life.


Now that's a little ridiculous, change the oil after every use? That's like
those people who change the oil in their car every 1000 miles. I've dealt
with enough 25-30 year old air cooled engines that were still in good
mechanical condition after obviously many hundreds of hours of use,
generally they had the oil changed occasionally but certainly not after
every use. Once a season is perfectly adaquate unless you're running 8 hours
a day, I change mine every year when I put it away for the winter.

I just checked in an old (1970's I guess) B&S service manual I have and they
specify oil changes ever 25 hours of operation so I probably end up changing
mine a little more often than that, but as you say oil does get combustion
byproducts in it, same reason a car driven all the time on the highway can
go a lot more miles between changes than one driven around town.

As for the lifetime ratings, the number of hours the warranty guarantees an
engine will last, and the number of hours it's expected to or actually will
last are two drastically different things, I would be absolutely shocked if
even the cheapest most shoddy lawnmower engine out there wore out sooner
than a couple hundred hours even if the oil was never changed (as is often
the case).