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SELECTION of the proper crude is an important consideration in the manufacture of aviation-engine oils. The authors class petroleum into asphalt-base, paraffin-base and mixed-base crudes, stating that scientific research and actual-performance tests have demonstrated the advantages of paraffin-base oils over asphalt-base oils for aviation engines, and that their superiority is now conceded by most authorities. Much attention has been given recently to the dewaxing and fractionating of lubricating oils, and this has resulted in an improvement in their quality and in their unrestricted use as “all-weather aero oils.” After quoting statements from several authorities who agree that an oil which will meet both summer and winter requirements is desirable, the authors give the definitions of viscosity, fluidity, consistency and plasticity determined by the American Society for Testing Materials and then discuss the fluidity or consistency of aviation-engine oils below their A. S. T.