Viscosity and density are key properties for the evaluation, simulation, and development of petroleum reservoirs. Previously, the friction theory (f-theory) was shown to be capable of delivering simple and accurate viscosity models for petroleum reservoir fluids with molecular weights up to similar to 200 g/mol and viscosities up to similar to 10 mPa s, under usual reservoir conditions. As a basis, the f-theory approach requires a compositional characterization procedure that is used in conjunction with a van der Waals type of equation of state (EOS). This is achieved using simple cubic EOSs, which are widely used within the oil industry. Further work also established the basis for extending the approach to heavy oils. Thus, in this work, the extended f-theory approach is further discussed with the study and modeling of a wider set of representative heavy reservoir fluids with viscosities up to thousands of mPa center dot s. Essential to the presented extended approach for heavy oils is, first, achievement of accurate P nu T results for the EOS-characterized fluid. In particular, it has been determined that, for accurate viscosity modeling of heavy oils, a compressibility correction in the way that the EOS is coupled to the viscosity model is required. Thus, in this work, the f-theory potential for the viscosity modeling and prediction of heavy reservoir fluids is further discussed.