This paper reviews some of the background which demonstrated that there were significant variations in the performance of commercial CD engine oils in highway truck and other heavy duty applications by the mid - 1970's. The paper reports that by 1979 service experience also showed that engine miles to overhaul were significantly improved when SAE 15W-40 oils were used in place of the earlier typical SAE 30 products. By 1983 it had been established that there was a 2 to 1 variation in miles to overhaul between the poor and the best 15W-40 oils.This service background was the stimulus for the development of a proprietary heavy duty diesel engine oil intended to provide customers with a product incorporating the best available engine oil technology. The initial objective was to maximize engine durability.The development of the Cummins Branded Oil was based directly on field tests of commercial and experimental engine oils in some 200 trucks which operated for more than 40,000,000 miles in internal engine company and cooperative field tests with oil and additive companies over a period of 3 to 5 years. Related commercial prototype oils also ran for similar periods of time in larger numbers of trucks.Life-cycle cost studies confirmed a marketing strategy which resulted in customers accepting the use of highest quality oils despite their higher selling prices. The use of these highest quality oils was to be a key factor in reducing total truck operating costs.