Change the hydraulic oil of a log splitter by placing the splitter on blocks, draining the pre-existing oil into a container, replacing the drain plug and pouring the new oil into the filling hole through a funnel. Six quarts of oil typically are sufficient to fill the reservoir to the appropriate level. Finish the job correctly by using the dipstick to double-check the oil level.
Never mix hydraulic fluids in a log splitter. Don't top off existing hydraulic fluid with some of a different hydraulic fluid; rather, drain completely if you do not know what lubricant is in the reservoir, and refill the reservoir with one type of an approved liquid. A full reservoir is only 75 percent full of fluid, and you need to flush the remaining air from the hydraulic cylinder. Do this by extending the piston rod as far as it can go and removing the oil filling plug. Quickly twist the release screw counter clockwise to cause the piston rod to retreat into the cylinder and expel the trapped air.
When choosing the type of hydraulic fluid, consider the temperature where you are using the log splitter. AW-68 is a common hydraulic fluid for use with log splitters when the temperature is above 50 degrees Fahrenheit.