oil question on fairly new engine build

First, hello all and new to the forum and the leaning tower of...get me where i'm going lol.

What are your thoughts on synthetic oil in the /6? The engine (225 in 66 dart) was completely rebuilt several years ago, broken in, then the car sat up for quite some time due to needing the complete brake system redone.

I am at the point of trying to get the engine going again, checking trans, etc. and if all is good then on to the brake system revamp.

Any advice on the oil viscosity and pros/ cons of going with full synthetic? Changing the oil (after trying to fire it up) is the next thing on my to do list.

Thanks in advance guys/gals!