As a lubricating oil deteriorates with use, contaminants in the oil build up over time. These contaminants include both chemical constituents such as oil oxidation products and acids due to combustion gases and particulate constituents such as dust and dirt from external sources, wear metal debris, and combustion soot A series of tests were conducted in a test cell to separate these categories of contaminants as much as possible and measure their effect on engine wear using Surface Layer Activation (SLA). A top piston ring, the corresponding cylinder liner and a sliding tappet cam follower were irradiated and the SLA measurements using the direct or marker method were carried out while the engine was running at close to full rated load and speed. In the absence of external contamination, it was found that organic contaminants and wear metals that built up over 200 hours of operation without oil filtration had no effect on wear of the irradiated parts. However, the unfiltered organic contamination caused the viscosity of the oil to dramatically increase. Low concentrations of graded SAE fine test dust was added to the sump in various concentrations to determine the effect of hard particle size and concentration. The quantitative relationship between particle size and concentration and wear rate was then determined and compared for different diesel engine families.