So what oil SHOULD be used with a new engine AND why?

Ottmundr said
Well, since FIAT owns Chrysler, you should run olive oil.

Service manual for my 68 Barracuda says 20w50 so that's what I run.

Royal Purple HPS 20w50. About $50 per case at summit. It does the job.

Their racing version is a bit more, but that is a good deal if it has the right zinc level etc.
I use Schaeffer's, they make it very clear...EXTRA ZINC!