The problems of changes in the properties of hydraulic oils during their use in hydraulic systems of wheel loaders have been presented. The methods and results of examination of selected characteristics of a hydraulic oil of the HV quality class and VG 46 viscosity grade as functions of oil usage time have been described. The test results have been analysed in the aspect of the predominating processes of the ageing of hydraulic oils and of the monitoring of oil condition. It has been found that the criteria of reaching the limiting condition of a hydraulic oil, specified in the literature and by oil manufacturers, are incompatible with the present-day hydraulic systems. The limit values as regards changes in the viscosity, viscosity index, and acid number are too restrictive. The predominating criterion of reaching the limiting condition by modern oils used in hydraulic systems is the content and particle size distribution of solid contamination.