Dave explains it really well, the only think I can add is about valve springs. Heavy valve springs and radical flat tappet cams are what get killed with low zinc levels. If you have a stock cam with stock valve springs, you won't have any problem. A 235 spring might have 60 or 70 lbs at rest and 120 or so full open, a good race spring starts at 120 to 140 installed and go to 290 to 320 lbs open. So you can see where 320 lbs of spring pressure is way different then your 110 lb or less springs. And once a cam has been polished to the lifter, is not nearly as critical to have as much zinc as a new cam needs.
On my 250, I can push the valves open just a bit with my fingers, the springs are just not that strong, so I run 10-30 oil with no additives. Dad and I have raced cars for years and have built many engines. We lost one cam a few years back before the word was out about reduced zinc. And like everyone says, it wipes the lobes right off the cam, and it didn't take but just a few miles once the fellow heard the rocker arm clatter a few times.
Joe
