Disregarding all the self proclaimed pundits on your vehicle, I've noticed the same using different oils. I'd speculate that the uniform molecules in synthetic might transmit vibration a bit better than the various sized molecules found in conventional oil. I'm sure there's been no testing in that regard, maybe I can get some money from the government to test my theory.
FWIW I've also noticed a difference in smoothness between 20 and 30 grades, in more than one engine. Also between 20, 30 and 40 grades in the (snif) 2013 F150 5.0 I used to have.