Hydraulic wood splitters, or log splitters, use hydraulic oil (sometimes called hydraulic fluid) that needs to be changed after every 100 hours of operation. In addition to changing the hydraulic oil, the level should be checked and maintained whenever you're using the splitter. Every 50 hours, you should replace the hydraulic filter. In any case, always clean the hydraulic filter at the same time as you change the hydraulic oil.
Remove the suction hose from the bottom of the hydraulic reservoir, unthread the inlet filter and clean it with penetrating oil. If the filter hasn't been replaced within 50 hours, discard the existing filter.
Let the hydraulic oil drain into a container and take it to a recycling centre for disposal.
Re-insert the existing or new filter and fill the reservoir.
Flush the reservoir with kerosene whenever you're servicing hydraulic components. It will reduce corrosion.