I seriously doubt anyone would have issues getting X amount of miles even on the cheapest store brand oil as long as oil is changed within reasonable intervals. Exception may be high performance motors where oil may see extreme temperatures.
Synthetic just gives you extra margin of protection that can tolerate much higher temps and extreme conditions with out breaking down or coking up.
I have seen where synthetics have gained hp over conventional oil on the dyno test. This was in an older pushrod mustang which also has more frictional losses compared to the overhead cam mod motors. But the gains were on the line of 1-2 % for changing all fluids over to synthetic so don't expect to see any real seat of the pants improvement.
Synthetic just gives you extra margin of protection that can tolerate much higher temps and extreme conditions with out breaking down or coking up.
I have seen where synthetics have gained hp over conventional oil on the dyno test. This was in an older pushrod mustang which also has more frictional losses compared to the overhead cam mod motors. But the gains were on the line of 1-2 % for changing all fluids over to synthetic so don't expect to see any real seat of the pants improvement.