My own perspective on using synthetic to extend oil change intervals is that doing so is not a good idea. While I respect that synthetics survive extreme heat better, such as that caused by turbochargers, synthetics are just as susceptable to particulate contamination and to dilution and contamination from combustion blowby as dino oils are. All engines have minute amounts of blowby, even new ones.
In short, synthetics are subjected to contamination just as dino oils are. If your owner’s manual recommends synthetics, then by all means use them. But if it does not, I remain unconvinced that under normal operating conditions it’s superior.
I also remain unconvinced that it should be used to extend oil change intervals. One of the oil’s many tasks is to wash down the cylinder walls, leaving a residue to lube the cylinders after the oil rings wipe them down. I believe that washes with oil carrying 5,000 miles worth of contamination are still better than washes with oil carrying 10,000 miles worth of contamination.
And now, my disclaimer. I have no data to back me up. I’ve yet to find any comprehensive studies that look at the long term effects of synthetic with extended oil change intervals versus dino oil with manufacturer-recommended oil change intervals. If any such study exists, I’d truely be interested in seeing it. Everything I’ve ever seen only compares performance of dino against sythetic in controlled lab experiments that may or may not represent real world use. And the lab experiments were all done in the name of selling synthetic.