Steve, you're correct about the zinc additive missing in modern oils. It's ZDDP or Zinc dialyldithiophosphate . This additive is not necessary in many modern engines because they run roller valve trains and there is not a lot of wiping action or side loading on the valves and cam lobes. In our old engines that do run flat tappets there is a lot of wiping load at the cam lobes and I'd highly recommend a heavy duty engine oil with this additive to prevent unwanted or premature wear. Some folks have differing opinions but using a 10w30 diesel oil in my TE-A20 has worked well for me and the oil stays clean.
I found this article on the subject. Take a look. Joe.