The current SN grade oil has about the same amount of ZDDP that the oil that was around when these trucks were new. If you're running a stock grind cam with stock valve springs, regular oil will be fine, synthetic would be even better. There's plenty of false information out there regarding oil and cam failures, most of it either from someone who doesn't really know what they are talking about, someone who had a bad experience with improper break in/improperly rebuilt engines, or someone who is trying to sell some sort of "miracle old car" oil by playing on the fears spread around.
Just for the record, I do run VR-1 in mine, as it does have a performance cam in it. However, the choice had more to do with the viscosity I'm running than any ZDDP fear. I run all of my other flat tappet vehicles on SN grade regular oil, and also put it in all of my customers vehicles with NO cam failures.
