In 50 years of driving, I've used nothing but conventional motor oils and with as many cars & Jeeps as I've had, I've yet to have an engine failure. The secret to good performance and engine longevity isn't whether it's synthetic or conventional, or has extra ZDDP, it is simply regular changes of the oil & filter. Heck you could use Walmart engine oil & as long as you changed it at the right intervals, your engine would be very well protected.
To hear some proponents of synthetic motor oils, you'll have ongoing engine problems/failures unless you use a synthetic which, of course, couldn't be further from the truth. And the ZDDP thing is not a factor for our low-tech Jeep engines with low cam lobe pressures. I would run a high ZDDP oil if I had a performance cam with stiffer springs but I sure wouldn't bother with a stock Jeep engine. Afterall, how many in the various Jeep forums are having tappet failures from the reduced levels of ZDDP (zinc)? Any at all? None that I have ever seen and I'm active in several Jeep forums and have been for 17 years now.
When would I pay extra for synthetic motor oil? If it was wintertime & I lived in some uber-cold area like Bismark North Dakota where synthetic oil's well known better flow characteristics are worth having. The rest of the year? I'd run a conventional 5W-30 or 10W-30 like from Valvoline, Castrol, Shell, Mobil, Havoline, etc.
One last thought... you can change back & forth between synthetic & conventional motor oils all you want. You can even mix them if you run low and don't have the same type to add... conventional and synthetic motor oils are 100% compatible. That you can't go back after switching to synthetic is nothing more than an old wive's tale.