The information I have comes from a number of experts in the field, most oil
companies and engineers at GM and Mercury Marine.  I hope they are facts and
not, as Larry so aptly said, "..too early for the facts to be  factual.."
The narrative below contains a good number of ", indicating that the facts
may not be factual at all.

I'm not sure anyone knows precisely what happens during "break-in", but the
idea is to wear off the peaks of the roughness mountains until the landscape
looks more like mesas surrounding oil-holding canyons.  Take too little off
and the engine might continue to wear, take too much off and there might not
be enough oil trapped in the surface.  I sort of believe the above
description.

So-called "mineral" oil lubricates "better" that dispersant oil because the
additives whose purpose is to keep contaminates in suspension are not good
lubricators.  The dispersant (detergent) additive takes up a significant
portion of the oil.  Presumably you need "good" oil during break-in to
prevent galling (one mountain welds itself to a mountain on the mating part,
pulling the mountain out by the roots, creating all sorts of mayhem).
However, in the last 80 years the technology of cylinder and ring surface
finishing has progressed to the point that it may not make much difference
what oil is used.  I definitely like Walter's method of running high power
and lean in order to reduce the peak pressure and spread it out over more of
the stroke.

I'm sure the requirements from Lycoming are good intentioned, but...if you
were the suspicious sort you could see a very strict break-in procedure as
one way to protect the manufacturer from us ("burns oil?  did you keep
mineral oil in for exactly 40 hours?  It was 39?  Sorry, we can't help.").
What incentive does Lycoming have to change their requirements?  And
finally, the auto industry has spent a lot of time and money perfecting
surface finishes that eliminate the need for any special treatment during
the break-in period.  The automotive and aircraft people do talk to each
other and the rings are made by the same people, so I suspect that strict
break-in procedures might have outlived their usefulness.  Except my engine
is so expensive, it doesn't hurt...and so it goes.

Gary Casey