Viscosity is among the most relevant properties required for the reliable and economic design of natural gas processing and transport units. However, the available viscosity data covering wide pressure and temperature ranges are scarce. This worldwide research project has a goal of producing accurate thermophysical properties for synthetic natural gas mixtures in a systematic manner using state-of-the-art techniques. The main purpose of the work is to analyze the effect of different compounds on mixture thermophysical properties and the validity of available predictive models. In this work, the viscosity measurements for the first two studied samples, QNG-S1 and QNG-S2, cover the temperature range (250 to 450) K and pressure range (10 to 65) MPa. Experimental measurements utilized an electromagnetic piston viscometer with a comparison to available reference data for pure methane. Using the reported results to probe the predictive ability of several theoretical models leads to unsatisfactory results. This work is one of the first available studies for the systematic analysis on the effects of natural gas components on mixture viscosity.