I have a brand new Gravely Pro Walk dual hydro walk behind. The manual doesn't state when to change the hydro oil and filter. When is this suppose to be done? I'm at 35hrs right now. I changed the oil and filter at 8hrs and plan to change it again at 50hrs and I always grease the fittings once a week as the manual says and blow out the air filter weekly and more often if I mow a dusty lawn. So I have it all down except when to change the hydro oil and filter!