Thanks, Steve.
A few facts:
Slant six engines are easy on engine oils. Open valve spring pressure is relatively low, especially on an engine with high mileage. High valve spring pressure can cause the metal to metal contact between camshaft and lifters. ZDDP in the oil is sacrificed to prevent this.
Modern engine oils have lower amounts of ZDDP, but certainly not none. Some are limited to about 2/3 of the amount present before it was reduced a couple decades ago. In a non-high performance application like a slant six that hasn't been modified, a quality modern oil will contain enough to give good service.
Quality engine oils from a known producer will contain other anti-wear additives to supplement the ZDDP.
Mineral base oil will do well in your application. To meet modern specifications however, most have at least some synthetic base oil blended in. Finding an oil with no synthetic base may prove virtually impossible, and not really worth the effort.
Many European auto producers require oils to be of very high quality, so good oil should be easy for you to find.
My recommendation is to find an oil from a well known company in the grade you like, probably a xxW-30 or xxW-40. If you can find a high mileage product, it will contain seal conditioners which may be helpful. Use it without additional additive and don't worry, it will perform well in your application.