Diesel engines are highly reliable, durable and are used for a wide range of applications with low fuel usage owing to its higher thermal efficiency compared to other mobile power sources. Heavy-duty diesel engines are used for both on-road and off-road applications and dominate the heavy-duty engine segment of the United States transportation market. Due to their high reliability, there are considerable numbers of on-road legacy heavy-duty diesel engine fleets still in use in the United States. These engines are relatively higher oxides of nitrogen (NOx) and particulate matter (PM) producers than post 2007 model year diesel engines. There have been various emission certification or verification programs which are carried out in states like California and Texas for different aftermarket retrofit devices, fuels and additive technologies for reducing NOx and PM emissions from these legacy diesel engines. During these programs, emissions from a candidate technology configuration are compared to a baseline condition using a representative legacy engine. However, under laboratory test conditions, a small variation in the measurement of emissions can lead to either passage or failure of the candidate technology. This research study investigates how engine oil aging history in a legacy engine affects the emission levels which may cause these decisive small variations.A 12.7L 1992 Detroit Diesel Series 60 legacy heavy-duty diesel engine was tested in an engine dynamometer test cell over 40 hours using commercially available Shell Rotella®-T SAE 15W-40 engine oil in two separate test campaigns. Gaseous and particulate matter (mass-based and particle number concentration) emissions were measured during the aging period. Oil samples collected at different aging stages were analyzed for changes in its physical properties along with ash content, soot contamination, and metal content. Additionally, the oil samples were analyzed for determination of particle size distribution of suspended particles in the samples using a novel technique.The results obtained showed no significant changes in regulated gaseous and PM mass emissions during either of the test campaigns that can be attributed to the lubricant oil aging within the first 40 hours. Analysis of the oil samples showed no significant change in viscosity or density within the aging time period, but showed significant change in total acid number (TAN), soot content, metal content, and total base number (TBN). Investigation into particle sizing of suspended particles in the oil samples suggested contamination of the oil by larger diameter particles during the initial 15 hours of its use compared to particles found from the oil samples from the later part of the aging time period. It was concluded that oil aging does not affect regulated emissions generation during the short time duration typically experienced during a fuel or retrofit device evaluation.