Lots of ways to deal with Zinc. Regular car oil + Zinc additive is good. If you need 20W50 that's a good place to start plus additive. If oil pressure is OK with lower viscosity, no need for the 20W50.

Another choice is an oil with high zinc to start with. I guess that is what Castrol recommended. Joe Gibbs, Brad Penn and others have this also.

My choice is diesel engine oil. Shell Rotella 15W40. This works for me and many others with flat tappet, readily available and cost effective. Pick your poison.

As far as synthetic, the 440 is no different from any other engine. Some additional risk of leaks if changing on a seasoned engine. Mobile 1 20W50 does have high zinc and would be fine with a flat tappet but would not recommend it in your case.