The aim of the study was to investigate the direction and intensity of changes in dynamic viscosity of engine oils during operation. The dynamic viscosity of the samples as well as of the fresh oil were measured at 150°C and two shear rates (128 s-1 and 256 s-1). Brookfield viscometer, model DV-II + Pro Extra, was used in the tests. The processes of degradation of five engine oils offered by different manufacturers have been thoroughly described. The quality aspect evaluated was the dynamic viscosity of oils used in cars of the same type, operated under the same conditions and over the same period of time. As part of the experiment, the changes in engine oils occurring during actual operation have been observed; the conditions of which can be described as "specific" (oftentimes harsh), i.e. significant and frequent changes in engine speed, sudden acceleration and braking, extended idling, driving in street traffic, short distance driving causing under-heating of the engine, and frequent start-ups at low engine temperatures. It has been observed that the change curves of the examined oils have a similar shape. All of the analyzed oil groups showed an upward trend in dynamic viscosity from the very beginning of the study, with the sharpest increase (10%) in the PS group, and the lowest in the MS group (5%) - at the shear rate of 128 s-1. When the shear rate was doubled (256 s-1), the PS oil group was still characterized by the highest viscosity increase (9%), while the lowest figures were found for the ME oils (4%). The results may facilitate decision-making regarding the service life of engine oils.