Tazza is right about the temps, heat is tough on the hydraulics. The manaul for my 5635 recommends ISO 46 that contains anti-rust,anti-foam, and anti-oxidation additives. They recommend Mobile DTE 15M, or Amoco Rykon 46. I use an Artic grade ISO 32 anti-wear because of the cold weather and it seems fine.When I change fluid I will change back to ISO 46 and see if the system gets quieter this summer.
they recommend "SAE grade motor oil " for the chain cases and I have been using a cheap multi-grade (Costco), again no problems. Also the drain plug for the hydraulic tank is at the bottom of the riser underneath the tank. scott