Silicone resins are hyperbranched macromolecules, which are nearly spherical in shape, nanometer in size, and covered with numerous terminal organic groups. For this reason, they can play a role of functionalized silica nanoparticles that are miscible with a polymer medium. In this paper, mixtures of linear polyisobutylene (PIB) with methyl-terminated silicone MT resin are considered. The phase states of mixtures were studied by laser interferometry method accompanied with rheological tests. It turned out that the solubility of the silicone resin in PIB matrix does not exceed a few percent even at high temperatures. Nevertheless, the addition of the resin significantly reduces the effective viscosity of PIB due to the shear-induced interlayer slip. To describe the concentration dependence of the blend viscosity, various empirical equations were considered, and their combination to describe the viscosity of blends in different phase states by one equation was proposed.