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 Signature Series 5w30 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)
AMSOIL Signature Series 5w30 - 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 5w30.
What's even more interesting is that this run with AMSOIL Signature Series 5w30 has been entirely on winter blend fuel, which should drop the efficiency even lower. There is potential that the AMSOIL Signature Series overcomes losses in efficiency due to the higher moly content.