FWIW the reason that so many new engines call for 5w 20 / 0w 20 has much more to do with EPA / CAFE mpg standards than engine tolerances. In the past 10 years or so there are multiple examples of the SAME engine going to a thinner oil with no other changes happening in that engine line. CAFE standards are calculated across the entire vehicle line. The tiniest 'gain' in efficiency for mpg can help raise the numbers for the manufacturer based on volume of vehicles.The tolerances in the newer motors are super tight so they need oil that can get in there hence all the 5W 20 and 0W 20 stuff. An pretty much all the new cars are running semi-synthetic or full synthetic from the manufacturer. Small block Fords were not designed around synthetic oils so its not necessary to run it. You can for sure run it as a lot of folks do and if you pay attention my guess is you will get more miles out of it and the higher cost is then moot.
I think it all stems from when people ran it too quick on a new build and the rings never set and they used oil. That went all over the internet and the "don't run synthetic oil" BS was born.
I do not run it because my engine builder told me to run the Brad Penn and with all the $$$ I have spent on these motors that is what they run.
One way you can see this easily - an engine used in both the U.S. and Europe, the SAME engine, will spec different oils from the manufacturer.

and to shut it off he had to pay GM for the software to delete it.