So I have been running an experiment. For the longest time I ran 5W-20 synthetic - typically Pennzoil Full Synthetic Platinum or Quaker State Ultimate Durability. In fact, the car has seen nothing for synthetic oil its entire life. I recently switched to Amsoil SS 5W-30 and I am surprised that there has been no change in mileage.
I have religiously tracked the mpg for the last year and here are the results:
5W-20 PP - 6112.5 mi tracked, 37.7 mpg average (includes daily driving and a trip to NJ in January 2020)
5W-30 Amsoil SS - 3342.4 mi tracked, 37.9 mpg average (includes daily driving and a trip to OH in Nov 2020)
You would think that if one were to keep similar driving habits you would see the 2 to 3% drop in efficiency, but I definitely don't notice a difference and therefore sticking with the 5W-30.
What's even more interesting is that this run with 5W-30 Amsoil SS has been entirely on winter blend fuel, which should drop the efficiency even lower. There is potential that the Amsoil SS overcomes losses in efficiency due to the higher moly content.