I use the Mobil 15W-50 that already has ZDDP (Zinc) in my 6.1 SRT Challenger ( No MDS )... And a few years ago Chrysler even recommended it for use in the 6.1 in warm areas such as Phoenix Arizona...
As for my 65 Cuda 273 Commando... I use Valvoline Premium Blue Heavy Duty DIESEL 15W-40 Engine (dino) Oil with the Friction & Wear Protection (ZDDP) already included... I love it as I don't want to be a chemist and it's also not pricy (but I still buy it when I see it on sale) @ Autozone etc... Comes in a one gallon container... Besides that... 15W-40 is a good oil weight for these 50 old engines... Maybe more so here in the Phoenix Desert area!!!