ABSTRACT Reduced operating costs and improved engine maintenance can be accomplished through scheduled monitoring of oil samples. Helmerich & Payne International Drilling Co. utilizes an oil sampling service as one method to reduce operating costs and to establish parameters for monitoring the condition of engines. The purpose of this paper is to discuss the cost effectiveness of implementing an oil sampling service, adherence to sample test results, and the development of parameters by which to efficiently operate the engines. Helmerich & Payne International Drilling Co. uses Caterpillar model D379, D398, and D399 diesel engines as the rigs' primary power source. Over the past four years, the use of an oil sampling service has increased the tiro between oil changes from the recommended 500 service hours to over 1000 service hours. Field data confirms that these extended intervals have translated directly into savings of $200,000.00 in the first three years of the program. As a result, H&P has established new guidelines for engine maintenance. The use of an oil sampling service has allowed H&P to reduce engine operating costs while maintaining its standard of optimum engine performance.