The paper deals with the results of cycloalkylation of phenol with 1-methylcyclopentene, 1(3)-methylcyclohexene in the presence of KU-23 catalyst and influence of various parameters on the yield of the target product. The reaction temperature was varied from 80 to 140 °C, the reaction time - from 2 to 8 h, molar ratio of phenol to cyclene – from 1:2 to 2:1 mol/mol, the catalyst amount – from 5 to 15%. Maximum yield of para-[1(3)-methylcycloalkyl] phenols is obtained under the following conditions: temperature – 110-120 °C, duration – 5-6 h, molar ratio of phenol to 1(3)-methylcycloalkene – 1:1 mol/mol and the catalyst amount is 10% based on taken phenol. Simultaneously, the yield of the target products – para-[1(3)-methylcycloalkyl] phenols is 68.6-73.5% per taken phenol, the selectivity is 91.8-94.3% on the target product. As a result of the chromatographic studies of the products of phenol cycloalkylation with 1(3)-methylcycloalkenes in the presence of KU-23 catalyst it becames clear that the alkylate mainly contains paracycloalkyl-substituted phenols (89.6-94.1%). After rectification of the alkylate at low pressure (10 mm Hg), the target products were obtained with a purity of 97.6-98.2% and their physico-chemical properties were determined. The resulting para-[1(3)-methylcycloalkyl] phenols were aminomethylated by formaldehyde and aminoethylnonylimidazoline at the ratio of 1:2:2. As a result, Mannich bases were obtained with yield of 68.3-76.7% from theory. The synthesized methylcycloalkylbenzylaminoethylnonylimidazolines were tested as antioxidants of M-8 engine oil. The oxidation resistance of the oil has been investigated without the addition and with the addition of ИХП-21 antioxidants, known benzylphenylamines and the compounds of the invention. Addition of the obtained compounds to the base oil leads to enhancement of its antioxidant properties, at application of which viscosity increase is 14.07-15.28%, and precipitate - 0.45-0.57%.