Preventative maintenance is a key component in maintaining fleet quality. Engine oil provides thermal management, wear protection, and corrosion resistance functions that optimize engine performance and life. Engine oil quality degrades through the result of chemical change and contaminants. The rate at which the oil degrades determines the frequency at which the engine oil should be changed. Engine oil is changed to combat contaminants and oil degradation, and to maintain a quality necessary to protect the engineThis thesis presents the results on an extended oil drain interval program on selected equipment, done to quantify engine oil degradation and contamination. Conventional mineral oil (Conoco HD Fleet Supreme® 15W-40) and synthetic oil (Rotella® T6 5W-40) were studied. The results show that the existing interval of 5,000 miles can be extended. The synthetic oil was tested on class 0210 trucks (6.4L and 6.7L engines). There is a significant difference in oil performance and chemical degradation between the 6.4L and 6.7L engines. The 6.4L engine oil had an approximate life of 8,000 miles, whereas the 6.7L engine oil had an approximate life of 12,500 miles. Class 0209 trucks operated using conventional mineral oil and had no significant chemical or physical changes up to approximately 6,500 miles. It was found that degradation rates for class 0303 and class 0311 tractors engine oil, both physical and chemically, occurred at the same rate. The intervals for class 0303 tractors were able to be extended out to approximately 250 hours and for class 0311 tractors were extended out to approximately 450 hours. Both classes of oil quality still measured well above the threshold limits.