The hydraulic fluid is a biodegradable oil. We are purchasing a 5-gallon container from a local oil company which is also biodegradable. Endless Pool Technical bulletin #25 describes the properties (viscosity, etc.) of their oil (Plantohyd 40N) which I copied and sent with hubby to purchase it.
We are just now installing a 2002 model pool (2nd owner - long story). The water pump and filter are working fine, just now bringing the hydraulic pump on line and we want to use fresh oil, as we found the shelf life of the bio oil is only a couple years.