Recently high copper concentration of used oils has been frequently detected in our oil analysis system, although any excessive wear of the copper parts in the engines and the power-train was not detected. We have found the copper comes from copper solder in the oil cooler. Thus we have investigated the copper corrosion characteristics of commercially available engine oils and power-train oils in a laboratory test. Most power-train oils did not show copper corrosion, however some engine oils showed large weight loss of a copper test piece. In a field test of off-road dump trucks, we have found that the copper, dissolved in the oil, caused glazing and friction reduction problem of the wet brake paper disk. To clarify the glazing mechanism of the disk, simulated bench tests and used oil analysis have been conducted.