If the manual book isn’t clear enough about the interval of replacing power steering oil, then always try to be on the safe side by changing the hydraulic oil when you change the lubricants in your vehicle for safekeeping. Also, check the OEM recommendation.\rIn general, do change every [100,000 – 11,5000] km or when you clearly feel that the power steering isn’t responding smoothly to your command. Also, check with a professional.