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The problems related to selection of motor oil bases were determined. The principles of altering the initial state of base oils in conditions of the high piston ring of the running engine were formulated and a scheme of successively occurring processes in the oil is reported. A method is proposed for evaluating the rate of response of the base oil based on the change in its state due to transformations in conditions of laboratory catalytic oxidative thermolysis.It is frequently necessary to not only expeditiously select a base of a required quality level but also to investigate the possibility of replacing the base from one manufacturer with a similar base from another manufacturer for manufacturing modern commercial motor oils for different user groups. These problems can be solved if not only the viscosity-temperature properties of the base but also its tendency toward catalytic oxidative thermolysis and responsiveness to different functional additives are taken into consideration.We will consider in general form a method for expeditious evaluation of the change in the state of base oils, for example, during thermal catalytic transformations in the conditions of use. Let us represent these changes by a scheme of successively occurring transformations:According to this scheme, oil, under the effect of high temperatures, passes from initial state A to metastable state B as a result of accumulation of acid products, and due to further transformations then goes to high-viscosity state C and subsequently to state D as a result of coagulation processes. These transformations usually change the overall state of the oils, characterized by complex index ΔS.Based on this representation, probability W ΔA of the change in the initial state of the base oil can be characterized by the product of the probabilities of successively occurring processes ____________________________________________________________________________________________________ I. M. Gubkin Russian State University of Oil and Gas.