Need advice on high ZDDP level non-syntehtic oil for my Dart 440

As long as you are using a high zinc oil I think the brand name is pretty much irrelevant. Check that it has somewhere between 1200-1400 ppm zinc and 1300-1400 ppm phosphorus and you're good to go (not recommended to exceed 1600ppm). You can have the discussion around synthetic vs dino oil that goes on forever. Personally I have been running 15w50 Comp Cams high zinc blended oil. Last case I bought from Summit I think came out to around $5/qt.