A capacitive sensor in its simplest form has three identical plated spatial electrodes, which are positioned equally spaced apart, in order, alignment and parallel with each other. The first and third electrodes are the outer electrodes and connected in parallel by a first lead wire serving as a first electrical pole to be a group of the integrated electrodes. The second electrode is the middle electrode and connected by a second lead wire serving as a second electrical pole. The sensor is applied with an alternating current (AC) excitation signal from an AC device. The present invention method includes application of referencing and sensing capacitive sensors to obtain a measured temperature compensated electrical property of the oil to thereby obtain a measured remaining usage so as to a predicted remaining usage of the oil. Therefore, the top oil level, or the normal, or the abnormal oil deterioration can be concluded according to the measured remaining usage which is respectively larger than, or similar to, or less than the predicted one of the oil.