Past work suggests that finer filtration of engine lubricating oil reduces wear caused by abrasive particles. However, field data confirming this is limited. Filters were tested in the laboratory to establish micron ratings and in the field to relate filtration and wear. Field tests were conducted on city buses powered by Detroit Diesel 6V92 engines. Oil analysis was used as an indicator of engine wear and as the basis for comparing different filtration systems. Used oil was analyzed for particle size distribution, wear metal concentration and oxidation. Lower particle concentrations resulted when finer full-flow oil filters were used. Further reductions in particle concentrations resulted from the addition of bypass filters to the filtration systems. Corresponding decreases in engine wear metals accompanied reductions in particle concentrations. Results from previous laboratory studies are supported; engine wear is reduced through the use of filters with higher efficiencies at smaller particle sizes.