This bachelor thesis describes the procedure for measuring the viscosity of an oil using a rotational viscometer. Through the paper basic properties of lubrication of machine elements have been explained (viscosity, viscosity index, pour point, flash point, volatility, oxidation stability). Also, other already developed viscosity measurement procedures are shown. On a rotational viscometer we need to determine the methodology for achieving and maintaining a certain temperature. This problem was solved with use of water flow through hose around the measuring sample. The experimental part of this assignment was performed in the Laboratory for Water, Fuel and Lubricants and in the Laboratory for Engineering Ceramics, both laboratories within the Faculty of Mechanical Engineering and Naval Architecture in Zagreb. The experimental part includes an analysis of three different engine oils which are available in our market: INA Super 5 SAE 15W-40, Shell Helix Ultra 5W-30 and Castrol Power 1 2T. Calibration fuel oil with prescribed parameters was used to calibrate the device. Calibration and measurements were performed at 20°C and 40°C. Each measurement was repeated three times and mean values were taken for final results. To compare the experimental results, calibration and measurement were performed on a Höppler viscometer. Some differences in the results were obtained in these measurements.