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

STP is simply an oil thickener. Chemically, it is polyisobutylene, or at least in the past it was. This thickens oil viscosity and probably raises the oil by at least one viscosity grade. SAE 30 becomes SAE 40, for example. It doesn't hurt anything but fuel economy but may increase the time to pump up oil pressure when very cold. Say -10 to -20°F. The added ZnDTP will not hurt though.