I work in the Heavy Construction Industry, we seldom change Hydro oil in our fleet of heavy equipment from a 2000 lb skid loader to a 150,000 lb excavator we do have a strict maintenance schedule which is every 250 hrs, engine oil, engine oil filters, air cleaner and hydro filters, and the engine oil is sampled and tested every 2nd or 3rd change, I guess that I got in the habit that when you change the engine oil and filter you change the hydro filter. I guess that I should mention that we run a lot of Hydraulic Impact Breaker attachments and no matter how careful you are when you switch from hammer to buck you get dirt/particals in the fluid. Hence the filter change. Actually now in some of our machines the manufactures have approved the use of 15w40 motor oil in the hydro systems so we only have do deal with one oil in the field... its nice...