The steady‐state melt viscosity of several plasticised hydrocarbon elastomers is shown to be described by a reduced variable treatment which allows superposition of the viscosity‐shear rate curve for any polymer containing a diluent on the viscosity curve of the pure polymer. The shift factors which accomplish this superposition may in turn be broken up into two factors: (1) a function of the volume concentration of the diluent alone, which represents the loosening of the entanglement network, and (2) a factor representing the effect of the diluent on segmental friction and chain configuration. The latter is shown to be related to the temperature of measurement and the glass transition temperatures of both polymer and diluent. The relationships developed permit predictions of plasticizer efficacy for any diluent/polymer combination from glass transition and density data alone.