Find the Hydraulic Filter Size
Hydraulic filters need replacing every 50 hours with a 10 micron filter. It's a good idea to clean it whenever you change the hydraulic fluid, or oil too. The filters come in different sizes based on manufacturer, and model and you should replace the filter with one of the same size. Finding the hydraulic filter size on a wood splitter can be accomplished by doing some research, or inspecting the filter and comparing it.
Open the operator's manual that was supplied by the manufacturer and look for a section related to maintenance of the hydraulic fluid and inlet filter. Look for the filter specifications that may be listed in that section. If the specification isn't listed, look for a part number. For example, Craftsman's 675 series operator's manual lists the filter as part number 723-0405. You can order that part without needing to know the size.
Look for a hydraulic fluid specification label mounted to the frame of the wood splitter. It's often on the frame adjacent to the hydraulic fluid dip stick. The filter part number will often be located on that label.
Remove the suction hose from the bottom of the hydraulic reservoir, unthread the inlet filter and physically measure the filter if the part number isn't sufficient and you need the actual physical size--for example, if you want to order a generic manufacturer's filter.