We use Mobil 1 synthetic 15W-50 motor oil per Hydro Gear because they consider it a lifetime fluid, so normally there is no need to change the hydro fluid. It is recommended to change the filter every 300 hours or at least once a year. Usually the only time the fluid is changed is for component failure, a leak that has possibly caused contamination or high hours. If the fluid looks clean and doesn't seem overheated there is no need to swap it, the least amount of times the system is opened the better. How many hours are on the unit?