An essential part of the total parasitic loss in an IC engine is due to the piston ring and liner friction. In this work, a piston ring-liner reciprocating test rig combined with ultrasonic film thickness measurement system was used to understand frictional characteristic of the lubricant that formed in the contact. Two test procedures were carried out for two lubricants with different viscosities. These procedures were a step load increment at a constant speed and a step speed increment at a constant load. The results showed that the piston ring-liner contact was in boundary lubrication regime for low operating speeds at high load. This was consistent with the oil film thickness data. However, mixed lubrication regime was observed for high operating speeds at low loads. The lubricant film thicknesses increased with speed and decreased with load. The correlation between friction and film thickness showed both the transitions in lubrication regime and the effect of viscosity and lubricant additive package. The experimental method is a first stage towards the measurement of oil films in a fired automotive engine.