An optical technique using laser-induced fluorescence (LIF) has been developed to measure the oil film thickness between a fixed point on the cylinder-liner and the piston of a single-cylinder diesel engine. Details are presented of an experimental layout for acquiring fluorescence data from the engine and a strategy for their conversion to oil film thickness.Engine tests have shown that the oil film thickness in the liner/piston contact depends critically on the temperature dependence of the oil viscosity and hence is sensitive to the engine's speed and torque output condition. Oil film thickness measurements were made for two fully formulated lubricants, a 15W/40 Universal Diesel Engine Oil (UDEO) and a 10W/30 Super High Performance Diesel Oil (SHPDO).From these oil film thickness measurements the analysis was extended so as to estimate the oil volumes present between the cylinder-liner and different portions of the piston as they pass the measurement point. The direction and magnitude of the nett oil flows were inferred from inter-stroke differences in these oil volumes. Significant oil volumes were transported up and down the liner within the engine cycle, however correlation of the “upward” transport with the known oil consumption for the engine was poor. This suggests that oil consumption is controlled by factors other than oil availability. Volatile losses and oil-mist formation within the ring-pack coupled with “reverse” blowby are believed to provide plausible routes towards oil consumption.