The effect of water contamination on the filtration of HD diesel engine oils has been investigated. A proposed industry laboratory “glassware” test was found to be sensitive to multi grade oils. Certain viscosity index improver chemistries proved more critical with some but not all detergent inhibitor systems. Experiments using a full-scale filter rig test, designed to simulate the laboratory test, confirmed the possibility of filter plugging. In the full-scale rig test however the oil's detergent inhibitor chemistry was found to be the major factor in determining performance.Extension of the studies to field engines indicated that it was difficult to produce filter plugging with water contaminated new oils. Further more oils which “failed” the laboratory and full-scale rig tests when new, would “pass” after a few hours of engine service. The results indicated that further work is required before bench tests can be established to predict filter plugging tendencies of water contaminated HD engine oils in service.