Temperature and pressure dependencies of shear and elongational viscosities were examined using rotational and capillary rheometers. Two different batches of the same polymer grade have shown that the molecular structure diversion significantly influences the magnitudes of pressure coefficients, which vary more than the temperature coefficients. The pressure effect on viscosity notably depends on the amount of long-chain branching in polymer Further the paper shows that pVT data analysis via the Simha-Somcynsky equation of state can be employed to reveal differences in temperature and pressure viscosity dependencies through the free volume fraction.