Monitoring pressure drop in an oil line has been proposed as a technique for assessing lubricant condition in an engine. This technique is evaluated using a commercial single cylinder, gasoline engine. The monitoring system consists of a differential pressure transducer and a thermocouple attached to a section of the oil line. Engine tests using single and multi-grade oils are carried out at a constant temperature (100 ± 5°C) and load (7 ± 1 N·m) over a thirty-six hour period. The pressure drop for single grade oil shows an average increase of 10% over the test period. This trend is similar for multi-grade oils over the first 20 hours. A larger rate of increase over the last 16 hours results in an average change of 50-70% by the end of the test. The corresponding change in kinematic viscosity is typically half of that for pressure drop. Differences between oils are associated with compositional changes in multi-grade formulations.