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. M. pour-points and the significance of dewaxing paraffin-bearing oils.Charts are presented and explained which show the effect produced on the A. S. T. M. pour-point by the addition of wax to a partly dewaxed paraffin-base oil, and the behavior of an oil at three different temperatures. A diagram of a fluidity machine is given and the results of tests made thereon are shown. Curves showing the comparative fluidities of three kinds of Grade 1 aero oils and an oil produced from a California crude; a similar series of curves for Pennsylvania and Ranger paraffin-base and for Coastal oils having viscosities corresponding to Grades 2 and 3 aero oil; and a series of fluidity curves for several oils from various crudes corresponding to Grade 4 Liberty aero oil are presented.In conclusion, oil consumption is discussed and the subject of the stability of an oil in service is treated. Other methods of testing are outlined, and the statement is made that the necessity for dewaxing paraffin-base oils and fractionating them to obtain a close-cut homogeneous product has resulted in the development and installation of new types of equipment for both the steam and vacuum distillation of lubricating oils. The authors say that these developments are of great importance for the development of all high-speed, high-compression, internal-combustion engines.Discussion of the paper by the first four members was oral at the meeting; the remainder is submitted written discussion. Various tests mentioned, the results of some of which are given, show among other indications that changes in apparent viscosity with changes in pressure, when correlated with engine-starting conditions, indicate that some definite relation exists between torque required and viscosity of the oil under high rate-of-shear conditions; that it is important to determine the viscosity at the correct pressure; that a power-driven Osborn testing-machine is a ready means of measuring the starting resistance of an oil and gave the same directional relationships that were found in engine tests; that an oil containing wax will be motionless until a certain shearing stress is applied and thereafter behaves like a viscous rather than a plastic substance.Data are given from an investigation of the problem of oil fluidity through an airplane pump-assembly, the results showing that the Furol viscosity and Greiner charts fail to indicate what an oil will do in actual service. One group of discussers question some of the statements made by the authors in their paper and disagree with them. The authors respond by citing their authorities and reply to the exceptions, concluding with the statement that automobile engineers and chemists have endeavored for some time to induce the American Society for Testing Materials to adopt a standardized volatility test, claiming that low volatility is a characteristic of a good lubricating oil.