FeCuC sintered steel is prone to rust, therefore rust preventive oil is used for storage. These rust preventive oils generally have a problem of reduced workability due to stickiness caused by high viscosity. However, few electrochemical studies have been conducted on the correlation between viscosity and corrosion resistance. Therefore, in this study, we investigated the effect of the base oil viscosity on the corrosion resistance of rust preventive oil by changing the base oil viscosity.As an experimental method, electrochemical measurements were carried out using the water screen test established in our laboratory, which can quantitatively evaluate the rust preventing ability. According to the polarization curves measurements, corrosion current density I corr is reduced by coating the rust preventive oil. It showed about 100,000 times better corrosion resistance with and without rust preventive oil. While, almost no difference in rust preventive ability was confirmed due to the difference in base oil viscosity. According to the results of the open circuit potential, the time required for the potential of FeCuC sintered steel to saturate was 505 ks for low viscosity, 651 ks for medium viscosity, and 340 ks for high viscosity. Therefore, it was found that there is no correlation between the time until rust occurs and the viscosity of the oil film. It was also found that the effects of the additives were fully exerted even if the oil viscosities were different. Therefore, the problem of reduced workability due to stickiness can be solved by using low viscosity base oil.