The effect of nitrogen-containing phenol oligomers on the viscosity-temperature properties of base motor oil is studied. It is found that the products of oxypropylation of phenol and monoalkylphenols (alkyl chain length -C(8)-C(12)) in the presence of benzoguanamine, as well as oxypropylated benzoguanamine have a positive effect on these properties. It is also found and scientifically validated that the viscosity -temperature properties of base-oil compositions are affected by the concentration, component composition, and thickening power of oligomers in oil.