Hydrogenated mineral oil was separated in steady-state conditions and in a solvent stream by thermodiffusion. In thermodiffusion separation, isoparaffins are concentrated at the top of the tower and naphthenes are concentrated at the bottom. Solvent stream separation ensures more efficient separation of alkanes from naphthenes: fractions with a higher content of isoalkanes than in separation in steady-state conditions are obtained at the top of the to tower. The dependence of the viscosity-temperature properties of thermodiffusion fractions of hydrogenated mineral oil on their group hydrocarbon composition is investigated.