In concentrated polymer solutions, the concentration (phi) dependence of the terminal relaxation time tau(d) reflects phi-dependent changes of several factors, the monomeric friction zeta(0)(phi), the entanglement length, a(phi), and the correlation length, xi(phi). Usually, the effect of the latter two factors on tau(d) can be cast in a simple power law, tau(d) similar to phi(nu). This power law form is to be examined for tau(d) after correction of the changes of zeta(0) with phi, but this iso-zeta(0) correction is an unsettled problem. The correction based on the concept of "iso-free-volume" has been attempted in literatures. This study focused on four groups of solutions with different local frictional environments to examine universal validity of this correction. The isothermal data of tau(d) were theologically measured, and then corrected to the iso-frictional (iso-zeta(0)) state. After this correction, tau(d) of most solutions in small molecule solvents showed the power law behavior tau(d) similar to phi(nu) with exponent of nu = 2.0 +/- 0.2, irrespective of the solvent type, either neutral small molecules or an ionic liquid (organic salt), and of the difference of the glass transition temperatures of the solvent and polymer. In contrast, the exponent became smaller (nu approximate to 1.3) for the solutions in an oligomeric solvent. These results are discussed within the frame of the two-length scaling theory that considers changes of xi and a with phi.