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The time dependence of the electrical properties for several fully formulated engine oils was tracked during engine dynamometer tests. The pattern for the resistivity indicates the existence of two types of processes (fast ion-depleting and slow ion-releasing) whose existence cannot be inferred through standard engine oil analysis. After these processes have subsided, the change in the resistivity correlates with the change in oil viscosity. We show that the peak value of the resistivity at early test times correlates with oil performance longevity as determined from conventional oil analysis parameters. In addition, the permittivity can provide an early warning of a dangerously low oil level. The significance of the results and their consequences for on-board monitoring of oil life through sensing technology based on electrical ac impedance measurements are discussed.