I just missed a deal off Ebay for one of those filter carts that run off AC, has two filters, hoses and quick connects. The idea here is to plug in to your unit when not in use and filter for several hours, or even over night. This will leave you with a clean tank of fluid. They say that most contaminates are introduced into the system through adding new oil, plugging in the PTO, and any crud thrown off by gears and particles of hoses, etc. The hydraulic guys say that you even filter new oil, that it does not meet some cleanliness standards. Here I am thinking that new oil was very clean. Have you ever taken off your oil fill cap on the hydraulic tank and heard that gritty sound? It has to breathe right. They even make oil cap filters that keep out moisture and dirt.