I think with 4 cycle engine oils it's pretty cut and dried that Synthetics are better.A lot of good full synthetic oils out there, and some very good semi synthetic oils as well. They all have benefits over dino oil, primarily the ash and carbon build up.
I too don't understand the mechanics hate synthetic oil thing. Been a heavy duty diesel mechanic from high school until recently. Done tons of oil samples and the synthetic oil 9 times out of 10 comes back that it was still good for continued use. Didn't matter if it was a Detroit 12v or a 5.9 cummins. The benefits ought weight any cons.
With two cycle oils it's less cut and dried. In fact some synthetic two cycle oils actually suck. It's also telling that JASO FD oils are almost all synthetic blends.