The North Carolina Department of Transportation operates a large and varied fleet of on-road and off-road equipment. Regular oil changes for these machines result in significant costs due to the required labor, replacement oil and filters, and disposal of used oil, as well as downtime for the machine. Provided that oil of sufficient quality can be maintained, PM costs can be reduced by extending oil drain intervals. The purpose of this research was to monitor oil quality throughout extended drain intervals to determine the type, rate, and magnitude of resulting degradation, and to investigate the potential for extending oil drain intervals. The oil analysis program established to analyze and monitor oil quality included selection of the oil analysis equipment, identification of threshold values for oil quality parameters, selection of NCDOT equipment for the program, and establishing oil sampling protocols. The OSA4 TruckCheck benchtop oil analyzer was used to analyze the physical and chemical properties of fresh and used oil samples of HD Fleet Supreme 15W-40 conventional oil and Rotella T6 5W-40 synthetic oil. Threshold values for measured oil quality parameters were established at conservative levels based on OEM recommendations, review of literature, and expert opinions.
A total of 952 samples of used oil were collected and analyzed from 47 machines that consisted of trucks in classes 0209 and 0210, and tractors in classes 0303 and 0311. Trucks in classes 0209 and 0210 were sampled at approximately 1500, 2500, and 5000 miles after the oil drain, while tractors in classes 0303 and 0311 were sampled at approximately 50 hour intervals. Machines on the extended program were sampled approximately every 1,500 miles or 50 hours beyond the normally scheduled oil drain. Analyses of the used oil sampled from the NCDOT equipment showed that the oils degraded chemically as the oil aged, but the observed viscosity degradation was not related to oil age. Contamination of the oil by water, coolant, dirt, or wear metals was not generally present. The results indicate that the oil drain intervals for most of the studied equipment can be conservatively extended. The economic and environmental impact of extending oil drain intervals for similar machines in the NCDOT fleet were estimated to be annual savings of over $120,000 and 2,500 gallons of used oil.
