similar to this using New York City Yellow cabs. Engines were new upon
start, then taken apart and analyized. Problem was that they did not
apply the same standard to all engines. I think that Mobile 1 engines
got a 25,000 mile oil change, so it's hard to compare if the engine
actually lasted longer.

I don't have a citation for you unfortunately.

What you described is exactly how this needs to be conducted. None of
this In My Experience stuff. That's how research works. Using high
mileage fleet vehicles is about the only way to really get a good
answer due to the time involved and the relatively small amount of
wear.

But I'd also be interested in seeing a cost/benefit analysis, too.
Synthetic costs a lot more than standard oil. If it costs 50% more, but
only provides 10% longer engine life, does it pay off? Even two of the
same cars and two of the same oils are likely to have dramatically
different engine lives. A significant sample is necessary, and that's
just not something I've ever found.