It is easier to use a single value to compare different polymers with each other.
Therefore, for the viscosity commonly the MFI (melt flow index) is used.
This value describes how much mass of a polymer is flowing during 10 minutes at a specific temperature and pressure (mostly 2,16 kg or 5 kg) through a given capillary.
That means the higher the MFI, the smaller the viscosity of the polymer.
The used unit is g/ 10 min.