The Perm State Microoxidation Test has been used previously to rate the thermal and oxidative stabilities of III C and III D automotive engine sequence reference oils. These ratings show good correlation with the results in the actual engine tests. In this paper, the oxidative and thermal stabilities of a series of heavy duty diesel engine oils are compared with III C and III D reference oils in the PSU microoxidation test using deposit level and overall production of high molecular weight condensation polymers (sludge precursors). These data show the heavy duty diesel engine oils to be more stable than typical III C and III D reference oils. The ratings of the heavy duty diesel engine oils in the PSU microoxidation test based on high molecular weight condensation polymer (sludge precursor) or deposit show excellent correlation with the ratings of the same oils in a 250 hour test in a heavy duty diesel engine.Test severities of the microoxidation testa run at Penn State and Caterpillar have been compared. The consistently more severe test results obtained at Penn State are within typical interlaboratory repeatability, and are probably due to a difference in fluid temperature during the test of 2° to 4°C.A comparison of diesel oil formulations at several temperatures show that the relative ranking of the stability of such oils will change with increasing temperatures. A comparison of mineral oil versus synthetic base lubricants shows that for the same relative behavior in the microoxidation test at 225°C, the rating of the synthetics improve relative to the mineral oils as the temperature is raised to 245°and 270°C.