If it's never been rebuilt then it likely was originally run with non detergent oil.
Switching to a detergent oil now has a great chance of breaking loose all sorts of dirt and stuff that was previously causing no harm sticking out of the way to engine parts.
Do you have a full flow oil system, or bypass?
There are a whole bunch of opinions on the subject I'm sure.. I was taught to run non detergent if that's what was originally used, and not risk breaking loose dirt that can cause engine problems. Otherwise if starting from a fresh clean engine then use modern oil (Shell Rotella SAE 30 is a good choice).
Grigg
