The objective of the work described in this paper was to compare the cold weather starting ability of diesel engines lubricated with SAE 15W-40 Petroleum and SAE 5W-30 Synthetic motor oil. Laboratory bench tests were used to compare rhelogical features such as borderline pumping temperature and cold cranking simulator profiles. A cold box provided a well controlled environment in which cranking and starting studies were carried out on the two oils in a turbocharged diesel engine. The SAE 5W-30 Synthetic exhibited higher cranking speeds, lower starter amperage draw and immediate oil pressure readings when compared to the SAE 15W-40 Petroleum. The SAE 5W-30 Synthetic oil was safely started at -10°F oil temperature without auxiliary heaters. The comparative field studies were carried out on six cyclinder turbocharged diesel engines representing conditions commonly found in the commercial and off-highway sectors. Basically these studies indicated that combining high capacity cold cranking amperage batteries, high pressure ether aid injection, and SAE 5W-30 Synthetic oil resulted in a system that safely starts diesel engines down to actual oil temperatures of at least -10°F.