They reformulated the oils, taking out the zinc and phosphorus (actually Zinc dialkyl dithiophosphate or ZDDP) when they found that it decreased the life of catalytic converters. It's not a coincidence that the OEM's shifted over to roller lifters and changed cam profiles at the same time the EPA dropped the ZDDP levels.
The problem with additives that you dose yourself is that the concentration of ZDDP is really quite critical - not enough and you're subject to high temperature breakdown of the oil, but too much and the zinc "plates out" or builds up on surfaces, causing galling. Here is a good, fairly short explanation that spells it out - https://www.thoughtco.com/debunking-...-legend-726162
You're much better off running an oil formulated with higher levels of ZDDP, like Brad Penn's original oil or some of the Valvoline Racing Oils, but know that you may start plugging your cat's over time. Google "Motor Oil ZDDP Levels" and you'll find a ton of discussion on what's best. Seems that the guys on the Corvette Forums have been talking about ZDDP for a long, long time and there are some smart guys there, keeping those old GM engines happy for many miles, but you'd be better off changing to a roller lifter cam.