The oil filler hole which is usually close to the top of the reservoir. Remove the plug and set aside, then fill the reservoir with oil. Make sure you don’t overfill or it can lock up the unit. Place the plug back in and you should be ready for work. Check the directions for bleeding instructions if there is air trapped in the jack. If fluid is leaking from any of the jack’s seals, stop using the jack until you get them fixed, otherwise the jack could fail suddenly at any time.