The evaluation of the kinematic viscosity and stability of distillate petroleum oil - residual petroleum oil, distillate petroleum oil - distillate shale oil and distillate petroleum oil - residual shale oil binary blends is discussed If is shown that when the low viscosity component is the petroleum originated light gas oil, the blend experimental kinematic viscosity is always higher than the calculated one, which is obtained using the standard blending calculation technique accepted for hydrocarbon oil blends. When shale oils, both distillate and residual, are blended with the so-called PTU-oil, an aromatic rich gas oil fraction, the blend kinematic viscosity can be evaluated by, the standard blending calculation technique. Contrary to this, the PTU-oil - residual petroleum oil blend kinematic viscosity is always lower than the calculated one. For evaluation the difference between the experimental and calculated values of kinematic viscosity, a simple equation can be used The dependence of the blend＇s stability on the volume fraction of components is studied.