I already told you if you hadn't changed the oil at all the engine would
have lasted 5 years. If you never even ever checked the oil it probably
would have lasted 3 or 4 years. Everything you said points to
lubrication failure. Next time why don't you try extra virgin olive oil
at least it will smell good when you burn her up.
-jim

We are talking about Briggs and stratton their castings are made in US.

Over-reinforce? We are talking about an increased machining accuracy of
only a few thousands. the amount of machining stock added to a casting
doesn't have much affect on the end result as it gets machined off.

And why would that be true? What would be the point of machining more
accurately if the worst gets worse and the best aren't as good? Sounds
like something right out of  Alice in Wonderland.

That's doesn't make sense why would less friction cause the cylinders
wear rapidly. There are a number of things that can cause modern push
mower engines to deteriorate rapidly. For instance, in an effort to
comply with cleaner air standards small engines are designed to run
leaner with no provision for mixture adjustment. That alone pretty much
guarantees they are going to go down hill pretty fast once they start to
loose a little compression. Also in the old days when the engine had
points if you hit something with the mower and the flywheel key sheared
a little the engine stopped running and you had to fix it. With the
modern ignitions the engine doesn't loose spark and can just continue to
run with the ignition out of time. Continuing to operate a mower with
incorrect timing can seriously shorten the engines life span. There are
other external components that are not as durable. But none of that has
anything to do with the quality of what's inside and in particular
nothing to do with the quality of the metal.

I believe Briggs and Stratton casts all their engines here in the US so
we don't need to muddy the waters with that little fairy tail. There ae
plenty of good castings made outside the US.

Steel engine block? - well that is a new one on me.

-jim

Whether you were aware of it or not - you ignored their recommendation.
And it would seem even now you prefer to preserve your ignorance rather
than find out what exactly what is/was recommended or why it is
recommended. Instead you prefer to make up facts out of thin air about
metallurgy and machining methods to explain what happened.

    Well, standard dino oil was not likely what they recommended for that
engine either. Whatever you put in the second time may have been even
worse than what you put in the first time. What originally came with the
engine was probably a synthetic. Briggs uses their own brand of
synthetic that they claim is especially formulated to withstand much
higher temps than other synthetics. And Briggs doesn't say use only
their oil (I don't think they legally can). What they do say is if you
choose to put 10w30 in the engine make sure you check the oil often.
These engines will run a long long time even when they are at the point
of adding a cup of oil with each tank of gas.

    If you had asked the dealer or any place that sells and services push
mowers with Briggs engines, you would find out that many people get
years of use from the same engine and they do no maintenance at all. In
fact, that is the targeted consumer for these cheapo mowers. They are
designed to be used and discarded and the engine will generally last as
long (usually longer) than the rest of the mower.
 
    It's likely that the carb has no adjustment for your engine so after
your rebuild it was probably running extremely lean which contributes to
making the engine run even hotter than it did before. Everything you
have said indicates that heat was the root cause of your problems. And
probably by the time you were done with your rebuild the grass was
getting pretty tall which doesn't make it run any cooler either. The
fact that someone else has an engine that doesn't run quite as hot and
thus survives proves nothing at all about what happened to your engine.
I suspect that there are thousands of purchasers that bought the same
mower at the same time you did and who have done nothing at all in
maintenance still have a running lawn mower.     
   
    But getting back to the original poster's question and my answer to
it.  If you had put what most people are using in their cars today (5w30
petroleum oil) the type of failure you experienced in that type of
engine would have been almost a sure thing. That's because the critical
temperature where that type of catastrophic failure occurs is lower for
standard 5w30 than it is for 10w30, which in turn is lower than a
synthetic like Mobil 1.  
    The OP didn't say what type of oil he uses. but in any case changing
the oil after a overheating incident is cheap insurance just in case the
oil reached the critical point where serious engine damage like you
experienced can occur. That doesn't mean that high temperature
definitely was the cause of your engine failure, but the facts as you
presented them, point to nothing else.

-jim