In my opinion there has never been a real life study to show which is more effective in producing the greatest longevity from an engine. I will bet that with 3000 mile changes between dino and synthetic there would be no difference. Now, 3000 OCI with dino vs 7500 or greater synthetic, maybe, maybe no difference either. Synthetics, IMO only permit one to go longer beween changes. No proof they extend the life of an engine, and, at what change interval as well.
I think a bigger advantage of the better synthetics is a reduction in engine deposits. It is this buildup of deposits over time, particularly in the piston ring and valve areas, that tends to degrade the performance of engines once they reach high miles. If you look at the benefit of AutoRX, it is primarily in removing some of these deposits.
I do think synthetics will provide lower wear rates under severe service conditions, or operating in very cold or very hot climates.
I have used Mobil 1 5W30 in my vehicles for over 15 years, changing between 5K and 7K miles (on avg, have gone 10K between some changes). I put over 140,000 miles on each of two of these vehicles (86 Honda Accord, 96 Nissan Maxima). Both of these cars were running fine when I traded them in. The Nissan's oil consumption was still negligable (never had to add oil before a change), while on the Honda I would have to add a quart after 3K-5K miles. At the time I felt tha the Honda's oil consumption was due to a change in driving route (I was down shift alot down a steep grade).
I use M1 synthetic for the extended drain intervals. I have the oil changed by the dealer when it is convenient. That is, I will wait until I also need a tire rotation, or new plugs, etc..