The rheological behavior of poly(ethylene-alt-propylene) (PEP), polydimethylsiloxane (PDMS) and polyisoprene (PI, two molecular weights: 70k and 200k) melts containing polyhedral oligomeric silsesquioxane (POSS) molecules was investigated by means of small angle scattering (SAXS and SANS) and oscillatory shear rheology. The dependence of the nanocomposite viscosity on the polymer-particle solubility and polymer molecular weight was studied. At high filler fractions all polymers exhibited hydrodynamic reinforcement of the plateau modulus quantified by a Guth Gold relation. Additionally, the PEP and PI70k systems showed a transition from liquid like to solid like rheological behavior. SAXS results enabled us to relate this behavior to the formation of a POSS network or POSS particle jamming, respectively. At low filler degrees, the zero shear viscosity of the nanocomposites was strongly dependent on the polymer solubility and entanglement number. We observed a viscosity decrease in the filled PDMS and PI200k samples, a constant viscosity in the PEP samples and regular reinforcement in the PI70k samples. These results are compared to the predictions of regular plasticization, and then quantitatively discussed in terms of the recently proposed model by Ganesan and Pyramitsyn, as well as the model by Wang and Hill. In particular the latter is shown to constitute a suitable means to quantify the results in terms of a layer of reduced polymer viscosity surrounding each nanoparticle.