The tribological properties of engine oils for heavy-duty trucks are evaluated, taking into consideration their variation during operation. After testing new oils or oils after a 2500 km and 5000 km run, there were no essential differences in their tribological properties at lower loads, but at higher loads and longer durations of operation, significant differences were found, including increased friction losses and the reduced surface wear protection ability of the oils. There are two main reasons for this reduced ability of the tested oils to form a boundary lubrication layer: the consuming of the functional additives and the aging of the oil, i.e., oxidation and an increase in acidity. Research data show a close relationship between the increasing acidity and surface wear.
