Can I use synthetic in a brand new engine?

I just rebuilt an engine for a 91 ford ranger 2.3L.
My manual says to use 5w-20 conventional but since there have been some advancements in the synthetic oils since the 70's, do I need to worry about seal swell or shrink? My instinct is to just use conventional to play it safe, but I'd really like to use synthetic.
I'm reading conflicted information…

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