5w-30 oil 40 years ago had a lot of ZDDP in it. Modern 5w-30 contains very little. ZDDP is essential for bucket type lifters or sliding finger followers. You may or may not get away with certain low ZDDP oils on a stock Z engine. Depends on the oil. But in a Race Car or a Weekend Warrior, with stiffer valve springs and a high lift , it can result in rapid destruction of the Cam lobes. We see it all the time on Audi VW forums and those engines have a lot less spring pressure and valve lift ( 5 valves per cylinder, 6 mm Diameter valve stems ) than an L-seriers.
Anything that is Energy Star rated is low content ZDDP. Thicker oils such as 20w-50, 15w-50, 10w-40 and Diesel oils are not Energy Star rated and usually contain sufficient percentages of ZDDP . It's all buyer beware and you have to really research oils these days for use in Vintage cars, Hot Rods or modified engines.
5w-30 was a recommended weight for Cold climates, but it is certainlt too thin for a southern States climate. These engines ar older designs with larger bearing clerances than a car fom the Mid 90's and up. Things change in 40 yaers.