The aim of this paper is to investigate the friction behaviour of modern automotive engine oils using a prototype block-on-ring tribometer under controlled laboratory conditions. The study focuses on measurements performed at constant normal load with gradually increasing oil temperature, representing the transition from mixed to boundary lubrication regimes, and on measurements performed at constant lubricant temperature with progressively increasing normal load until lubricating film breakdown. Eight engine oils covering SAE viscosity grades from 0W-8 to 10W-60 were tested. Friction torque was measured at defined temperature steps and used to evaluate the coefficient of friction for each operating condition in the constant-load sequence. In the increasing-load sequence, friction torque was monitored as a function of the applied normal load until seizure occurred, enabling the assessment of the load-carrying capacity of the lubricating film. The results indicate differences in the friction behaviour of the tested oils depending on temperature, load, and viscosity grade. The proposed experimental approach enables a reproducible comparison of engine oils under controlled conditions and represents a suitable tool for laboratory evaluation of their tribological properties.
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