I agree with greg g and thrashincows.
The oil subject seems to be a never ending theme. I'll add my 2¢
Flat tappet lifters, either hyd or solid, require the lubricity provided by zinc and phosphorus. No more, and no less complicated than that. Will they, do they and can they survive without alot of zinc. No doubt.
Diesel service oil? Oil rated for diesel is slightly different than oil rated for gas service since the diesel rigs are considered severe duty. But, last I checked, diesel engines still used bearing inserts of 'similar' design to gas engines inserts, and still require that the oil provide the cushion, lubrication and cooling to protect the various components that need oil to survive....just like a gas engine. My wifes Durango has had a steady diet of the same 15-40 that goes into my 5.9 Cummins and at 145K it still runs fine. My flat tappet engines get oil with zinc or a zinc additive and I'm not a brand snob. My experience says poor maintenance and short driving cycles (not up to temp) cause most oil related problems. You get to define 'poor' maintenance.