Synthetic oils of polyol ester, polyalphaolefin, and diester type were tested in heavy-duty diesel engines used in agricultural applications. Evaluation included dynamometer engine and field tests. The synthetics gave good oxidation resistance at high temperature and long drain, and exhibited low oil consumption. Additional testing and a long-drain engine oil classification are necessary to quantify the extended drain interval capability for synthetic oils under a wide range of operating conditions.Synthetic oils appear to be in a position to readily take advantage of future additive technology development. This fact, along with potentially more stringent temperature requirements for tomorrow's fuel efficient diesel engines, may result in an increasingly important role for full and partial synthetic lubricants in the future.