The basic goal of this research is to find the optimum drain interval by using oil analysis methods and techniques. Chemical and physical analyses of used mineral crankcase oil were used to estimate the condition of the lubricant in five Diesel commercial vehicles servicing for a parcel service in Turkey. The main goal is achieved by the help of calculation of the overhaul engine and oil costs per interval related the iron wear element concentration value in the sample oil. Lubricant samples were withdrawn once nearly every 3000 km for wear metal analysis and the evaluation of the deterioration of the lubricant. Finally, the optimum oil drain interval tor three Mercedes Sprinter 308 CDI's and Peugeot Boxer 2.2 HDI is found as approximately 28000 km, and for the Fiat Doblo 1.9D as 25 000 km. As the accustomed oil drain intervals are 20 000 km for Mercedes Sprinter and Peugeot Boxer and 15 000 km for Fiat Doblo, the nationwide extra cost paid for the lubricant over the lifetime of these vehicles would be, respectively, $4 429 425, $16 730 775 and $44 708 640 taking into account the number of those vehicles in Turkey.