I’ll add that regardless of the oil being used, maintaining ~1300-1350ppm zinc/phosphorus (ZDDP) in the oil is highly recommended in insuring adequate anti-wear additives for the flat tappet camshafts and lifters. There are a number of synthetic oils out there that do have the correct amounts of ZDDP in the lower viscosity grades such as Amsoil but you’ll have to search them out. You’ll find that these particular oils are not API approved but that’s simply because many times they are made for a specific application and simply fall outside of the API specifications. As Gary brings up, all 'SM' oils with '40' or higher in the labeling will have sufficient amounts of ZDDP for the older engines.
Here are just a few of the past threads covering the oil subject. Using the search function will bring up more.
Detegent versus non-detergent
General discussion 1
General discussion 2
General discussion 3
General discussion 4
Gil Baumgartner oil study
Multi-viscosity oil clarification
What is ZDDP?
Why to not use diesel oil in a gasoline engine
ZDDP