In an environment where glycol-based hydraulic fluid flows at high speed through a hydraulic circuit, the seat of a linear solenoid valve wears due to the synergistic effects of cavitation erosion and corrosion of the metal surface caused by the hydraulic fluid. The wear of the seat of a linear solenoid valve causes leakage of hydraulic fluid in the circuit, so there is a need to elucidate the wear factors and to reduce the wear. Focusing on the additives in the hydraulic fluid, tests were conducted in which hydraulic fluid containing different additives flowed in a hydraulic circuit. As a result, it was found that the hardness of the sheet surface after the test differed depending on the additive contained, and that the amount of wear was affected by the additive. The results of surface observation by FE-SEM and elemental analysis by EDS showed that the wear patterns were different depending on the additives contained.