Mathematical model of the measured relaxation electric parameters of engine oil is designed. There are two subsystems of measuring conversion in the model: function of capacitance conversion and function of resistance of transition processes durations. In the result of physical realization of the model, own spurious capacitance and conductivity of mockup are experimentally determined depending on temperature in conditions with no external measurable parameters. The use of elements with optimal parameters in the circuitry of the measuring transducer minimized the influence of spurious conductivity. This made it possible to reduce the measurement error of the relaxation electric parameters of engine oils, increase the sensitivity along the resistance channel, and practically eliminate the mutual influence of both measurement channels. In addition to the measurement channels, a Brown filter is used to separate useful information from noise.