You'll find plenty of opinions on zinc amounts, weights needed, synthetic or conventional oil, Ginger or Mary Ann etc.

If the PO has run 10W30 synthetic for the last who knows how many years  without wiping out the cam lobes, that should either make you scared that it's gonna happen next time you turn the key or make you say "maybe it's OK"

One pretty much universally accepted oil for these engines is 15W40 Shell Rotella conventional diesel oil.

It's reasonably priced and it's everywhere

You can also find it in synthetic or synthetic blend if you want that.

You can also substitute your favorite brand of oil in the same 15W40 diesel rating and be just fine.

They all have enough zinc to keep the "gotta have lotsa zinc crowd" happy.

There's no real magic number for zinc concentration that is gonna cook the engine if your oil has less than that number. Your PO has done a good job of showing that already.

Everybody has a favorite, the Mobil 1 15W50 mentioned earlier works for plenty of people as one of their favorites. 

So................  it's had a lot of years before that with some unknown oil  

What weight do you want to use that you're used to ?

You can find plenty of different weights of Rotella with plenty of zinc these days.