Systems and methods for automatically evaluating the useful life of an engine oil based on data corresponding to a plurality of preselected key oil properties. The evaluation includes determining, in connection with each of the pre-selected key oil properties, a first current measured oil property value, a first reference oil property value, a first deterioration-limit value, and a first weighting factor. The evaluation also includes determining a single index value, indicating a state of deterioration of the engine oil, in a calculation using each of the values and factors.