4 1/2 or 5 quarts of oil with 1/2 quart of ZZDP?

As 65TerrorCuda said, putting in aftermarket additives may not be a good thing. A little extra ZnDTP probably won't hurt, but engine oils are extensively tested to prove performance under extreme conditions and have to both protect from wear and keep the internal parts clean. That is why oil has to be drained. Don't believe the "Thermal Oxidative Breakdown" claims. That only happens when you let the oil stay in the car way too long. I have many horror stories from Chrysler lubricant engineers about turbo failures in 2.2L engines (back in the '90's) due to no oil changes.