Don't forget the A has a splash and drip oil system - not a pressure one. The 40 may be too heavy to easily lube the rod bearings and flow too slowly to make the mains happy. A looser engine could use the 40 weight to better cushion the babbit. Longer lasting insert bearings should probably stick with 10-30w. I'm not sure if the cost of synthetic is justified at the recommended 500 mile change rate. And yes the valve spring pressure in an A is so low you don't need to add zinc.
Understanding viscosity -
https://blog.amsoil.com/understanding-oil-viscosity/
New engines have tighter tolerances and were designed for oils like 5-20 - mainly as an fuel economy move to get the EPA numbers up. When the Chevy ZZ4 high performance crate engine came out a lot of people felt the factory recommended 10-30 was too light so they went with straight 40 and suffered bearing failure because the rod side clearance was very tight and would not allow the oil to flow out fast enough so the oil stayed around the bearing too long, got hot and burned causing the engine to fail.