Engine lubrication troubles resulting from the dilution of the lubricating oil in engine crankcases appear with increasing frequency, particularly where economy demands the use of cheap grades of fuel. Unless a lubricant not miscible with present engine fuels can be produced, lubricants will steadily decrease in viscosity whenever fuel finds its way into them. The most satisfactory remedy is to prevent dilution of the oil. To prevent absorption of the fuel by the oil is a problem of engine design. In experiments made by the Bureau of Standards the absorption of fuel vapors at average engine temperatures was found to be negligible; further experiments and oil tests showed no indication of dilution due to cracking, with representative refiners' products from typical crude oils available in this country. Information was necessary relative to the effect on the viscosity of lubricating oils of their dilution with varying percentages of fuels; the maximum temperature to which they can be raised without serious deterioration when exposed to the air and to their own vapor, and the maximum temperatures to which they must be heated to remove the necessary proportion of the fuels they contain. A series of experiments made to determine the effects of fuel admixture on oil viscosity is therefore next considered and this statement is supplemented by additional data and discussion relative to distillation methods for removing the fuel from the oil. Several further experiments were carried out to indicate possible improvements through simple changes in distillation methods, relative to time of heating and area of free surface exposed. The ultimate solution of the problem is believed to lie along the line of new departures in engine design that will prevent the accumulation of liquid fuel in the cylinders and in the lubricating system. In an appendix suggestions are made advocating the use on existing engines of some self-contained device for restoring the viscosity of lubricating oils, with some details regarding the features such a device should embody.