The higher FER value should correspond to a higher level of oxidation. The obtained FER (Eq. (1)) is compared with a predetermined threshold, and if the obtained FER is below the threshold, then the oil is in good condition; otherwise, the oil condition is unacceptable. This method was used to evaluate the degree of oil oxidation level, and in this measurement, absorption of optical radiation by the oil and temperature influence on test results do not need to be accounted for