I'm certainly no expert so wait 'til more folks respond but it seems like when people ask what to put in older engines and/or engines with flat-tappet cams, the general recommendation is to use an HDEO or something like Amsoil AMO 10w-40. Those types of formulations have more ZDDP I believe. Alternatively, there are ZDDP additives that you can add to newer formulations but you'll have to research it a bit. Given that you're in Texas, I doubt your start up wear will be much different with 0w, 5w, or 10w and I would worry more about using an oil designed with that type of older engine in mind. It's also entirely possible that something like Amsoil AMO might flow better at a given colder temp than a dino 5w. For example, GC which is a 0w is actually thicker at some temps than some 5w.