In this paper, we report on how interaction strength varies with pressure and temperature for several polyolefin mixtures. We find that the interaction energies that govern phase behavior in polymer blends are only a function of density for UCST polyolefin blends far from a critical point. As a result, the effects of pressure on miscibility can be predicted for such blends from knowledge of the effects of temperature on the interactions combined with PVT data. This remarkable simplification appears to be related to the van der Waals nature of the interactions between saturated hydrocarbons. Density dependence predicts the trends correctly for LCST polyolefin blends, but for these mixtures the interactions depend in a more complex way on T and P.