Four engine oils were field tested in three different types of truck diesel engines using 100,000 mile drain intervals. Periodic oil samples from each test vehicle were analyzed. After 100,000 miles engines were dismantled, inspected and rated. Results of this test work indicate that drain intervals much longer than currently recommended are possible with current engine and oil technology. Also, filter change intervals can also be extended from current practice.Included in this paper are analytical data from the used oil samples and engine inspection reports for the four oil formulations tested.