The antique design (through 1930's) engines with actual flat tappers that had wear problems now attributed to anything OHV were solid (not hydraulic) flat bottom lifters, and had splash/drip lubrication of lifter/cam. That means the cam lobe to follower bottom interface has no pressurized oil feed going there, through the cam. You will rarely find a design with hydraulic lifters that are not pressure fed, or that are actually dead flat.
The engines with the actual non hydraulic and non oil pressure fed (drip/splash only) cams with actual flat bottom followers generally will need cam/lifter replacement at anywhere from 30,000 to 50,000 miles, depending on driving style if the engine is in factory stock (low compression) configuration. Raising compression ratio or adding a supercharger will obviously aggravate this problem. Replacement cams eventually began using (literally) "fatter" cam lobes with more realistic profiles, and those would last much longer, and that added longevity was often attributed to some whiz bang oil.
The engines with the actual non hydraulic and non oil pressure fed (drip/splash only) cams with actual flat bottom followers generally will need cam/lifter replacement at anywhere from 30,000 to 50,000 miles, depending on driving style if the engine is in factory stock (low compression) configuration. Raising compression ratio or adding a supercharger will obviously aggravate this problem. Replacement cams eventually began using (literally) "fatter" cam lobes with more realistic profiles, and those would last much longer, and that added longevity was often attributed to some whiz bang oil.