Please see the diagrams for a typical service jack and a typical bottle jack.
With the ram fully retracted, locate and remove the oil plug. Open the release valve and pump the handle six to eight times; this will release any pressurized air that may be trapped in the system.
For service jacks the correct oil level is 3/16” above the inner cylinder. For bottle jacks the correct oil level is just below the oil filler hole with the jack sitting upright. Make sure the ram is fully retracted before checking the oil level.
Only use a good grade of hydraulic jack oil. This oil is available at auto parts stores and most retail stores with an automotive department.
Try the bleeding procedure ( #2 ) and check the oil level ( #3 ).