ZDDP is lower now in SN but oils have other EP chemistry today. I own many old GM muscle cars and never had a cam issue. It's all about intended RPM range. That dictates spring pressures required. Given our working RPM range, the GMC Olds is one of the last candidates for high ZDDP oils. Remember that ZDDP increases general friction in engines , but does act as an EP lube as an accidental secondary benefit. It was first added as an antioxidant with the accidental discovery that it was an Extreme Pressure lube. Same as Viagra, which was a heart drug that failed clinical testing phase, but had secondary effects.