I have never used synthetic oil in anything. "Synthetic Blend" advertised as 50% synthetic is kind of a hoax, as normal motor oils these days are about 25% synthetic anyway.
I have a theory that lack of ZDDP is irrelevant, because when they remove ZDDP they add something else to serve the same purpose. It is almost impossible to buy a bad motor oil these days, since the quality is mandated by the auto manufacturers.
I also like to put my money where my mouth is, so about 20 years ago I started using the cheapest oil available at Walmart. It is their house brand "Super Tech" 20W50, and I have good results with it for hundreds of thousands of miles. A while back someone else decided to give it a try, and they looked up the spec's for it, and reported that, "It has plenty of ZDDP". Say what?
So I looked up the spec's, and sure enough it has 1300 PPM ZDDP. So now I have egg on my face, because I didn't prove that you don't need ZDDP. But I did show that you don't need to spend big money an anything special to get good serviceable motor oil.
As so happens, much of the ZDDP was removed from SOME oil to protect catalytic converters. Catalytic converters were introduced for the 1975 model year. Any car built after 1974 (with a CC) uses oil no heavier than 10W40. So they didn't have to remove the ZDDP from heavier oils. So it turns out the heavier oils, 15W40 and 20W50 never had the ZDDP removed, and it is apparently an industry wide standard. So when I buy the cheap oil at Walmart it has the full dose of ZDDP.
The engine currently in my MGA has over 300,000 miles on the crankshaft and main bearings and rocker shaft and rocker bushings. Only reason the rod bearing were changed is because of loss of oil from a failed oil cooler hose last Spring. Cylinder walls and camshaft and tappets have over 200,000 miles and still doing well. Pistons were changed again last Summer (after 200,000 miles) due to a broken top ring, but the cylinder walls are still okay, so I just honed it lightly and dropped in the new pistons and rings. I had to change the cylinder head four years ago, but the old one had been on three different engines in 20 years, something like 300,000 miles.
Cheap oil is good? Food for thought.