Over the Counter Zinc Additive

Quality of oil vs break down plays right along with what zinc actually does.... cushion parts. A high quality oil (not brand name, but synthetics) will also provide protection as well. when oil quits doing it's job, look out camshaft !! Heat, along with mileage, begins the breakdown of oil. Better quality oils will stand up against this breakdown longer and offer better protection.
I myself, in my 318 Duster, use QS high mileage (it has a little synth. blend and a little higher zinc count), and then I add a bottle of ZDDP MAXX. It's 14.97 for the 5 qrt jug of QS at Walmart, and 8 bucks for the ZDDP additive. 23 buck for oil that protects, not bad I dont' think