The relationship between weight averaged molecular weight and zero-shear viscosity (commonly known as the Mark-Houwink equation) as determined by dynamic oscillatory rheometry was found to depend upon the absence or presence of viscosity-modifying additives such as a plasticizing oil. An empirical expression was derived to estimate molecular weights from zero-shear viscosities and compensated for the presence of plasticizing OIL or nucleating agent and their influence on the viscosity measurement. The model was verified by calculation of weight-averaged molecular weights of irradiated polypropylenes, and comparison to molecular weight determined by gel permeation chromatography. In the process of derivation, it was found that superposition principles and interpretations of viscosity curves based on pure polymer systems were equally valid when applied to viscosity-modified polymer systems.