In order to increase service intervals and reduce the amount and cost of waste disposal, many heavy duty diesel engine operators are investigating the technique of continuously blending small amounts of used lubricating oil into the fuel system for burning while replenishing the sump with fresh oil. The purpose of this study is to investigate the effect that this practice would have on erosion in fuel injectors and, additionally, to establish the level of fuel filtration efficiency needed to prolong injector life. Surface layer activation was used in an 11 liter diesel engine to measure erosion rates during engine operation as concentrations of used oil from 0 - 4% were blended into the fuel. A spot between the spill ports in an injector barrel was irradiated as a critical erosion location and the engine was run near full rated power for 24 hour test increments to establish erosion rates. Levels of fuel filtration tested on-engine included no filtration, a convention cellulose filtration medium, and a high efficiency, multilayered, synthetic medium. Fuel / oil blend cleanliness was determined using a screen obstruction technique. The effects of oil concentration and fuel / oil cleanliness, as affected by fuel filtration, were established and the test cell results are correlated with field experience.