In recent years, the varnish caused by turbine oil oxidation products has become a serious problem in thermal power generation plants. In our laboratory, we have developed a varnish diagnosis method that uses membrane patch colorimetry for evaluating varnish potential. Membrane patch color is influenced by filtering conditions such as mixing solvent, oil temperature, and incubation period. It is important to understand the influence of these conditions on the patch color and oxidation products to properly evaluate oxidation products using membrane patch colorimetry. Hence, this study aims to investigate these conditions by using two experiments. First, we conducted a filtration test on sample oils that were filtered under different conditions, in which we altered the mixing solvent, oil temperature, and incubation period to investigate their influence on membrane patch color. Then, we investigated the behavior of oxidation products versus heating time, incubation period, and oil temperature using an in-situ analysis of a microscopic fourier transform infrared spectroscopy (FT-IR). In the filtration test, the membrane patch color became darker with increasing incubation period. Additionally, the results also indicated that the membrane patch color became brighter when the sample oil and solvent were mixed and/or the oil temperature was high. From the in-situ analysis using the microscopic FT-IR in the case of a heating process, the peak of 1710 cm-1, which is an absorption band of carboxylic acid (-COOH), shifted to a higher wave number. In the case of a holding process, the peak of 1710 cm-1 shifted to a lower wave number. Overall, the results suggest that the oxidation products became low-molecular owing to cleavage of hydrogen bond and easy solubility in turbine oil when the oil temperature was high, and that the oxidation products became high-molecular by hydrogen bonding and difficult to dissolve in turbine oil when the sample oil was cooled and stored. Moreover, from the behavior of the oxidation products at different temperatures in the in-situ analysis, the cleavage of hydrogen bond of oxidation products in turbine oil ended after reaching at least 60–70°C.