Hi, Tom,
I have a 347 also, built for me by Proformance in New Jersey. The engine came with Gibbs 15W-50 motor
oil, and the builders recommend that or 20W-50 or 15W-40. I use the latter grade in Chevron's Delo as I have several diesel engines to maintain and it keeps stockage simple. The Delo--and Delvac for that matter--has plenty of ZDDP, so you don't need to add it.