More than five years' experience in the field, including two years of commercial application, have demonstrated the ability of a new class of lubricants to handle engine applications where even the best of the heavy-duty type oils were unsuccessful. The Special Heavy-duty oils, also referred to as “Caterpillar, Series 2 Oils” and “Ordnance, Supplemental List 2 Oils,” have been particularly outstanding in diesel engines where difficult fuel and operating conditions prevail. Practically all corrosive wear and engine fouling in such diesels has been eliminated. Unsatisfactory experience with these oils in such applications is almost unknown. The new lubricants offer the possibility of economical use in many diesel engines over and above the economics to be realized through reduced maintenance. Where oil-change period can be adjusted to fuel and operating conditions, the long-lived Special heavy-duty oils normally will be more economical than any of the ordinary heavy-duty type oils. There are definite indications in natural gas engines that the Special heavy-duty oil is of benefit in extending engine life. These engines are often limited by piston lacquer, and in those cases reduction of lacquer leads to more satisfactory operation and longer life. Work in the gasoline engine field to date has indicated that the Special heavy-duty oil performs at least as well as other types of lubricant. Definite advantage can be obtained as to engine cleanliness with the new oil. Excessive combustion chamber deposits leading to rapid increase in octane requirement and short spark plug life have not developed. Exhaust valve deposits have been observed, but there is no clear record that they adversely affect valve life. Our conclusions at this time are that, while gasoline engines do not require an oil of this type, some advantage may accrue to its use in certain types of service, and there seem to be no limiting factors.