Step 18: Fill with fluid
I used about 20 ounces of jack fluid. Around 12 ounces was used to fill the jack. The rest was used for cleaning and flushing the bottle jack unit. The fill hole is 1/4 inch in diameter. Even though the bottles for the jack oil have a pointed end, some spurts out while trying to get the bottle end to the hole, and it makes a mess. Get a funnel with a very narrow end. This is one I made specifically for this job from some sheet metal.
I filled the reservoir in small steps. The jack's release valve was open. Occasionally I pumped the yoke for the handle. When oil was at the level below the plug hole, I pulled the lift arm up and let it fall two or three times. This is to draw oil through the jack. I pumped the yoke between five and ten times to remove any air lock in the check valves. I checked the oil level again a couple of times. Then I returned the reservoir cap. The reservoir cap appears to be open, but actually has a small felt filter inside it.
Update: After using my jack a few times, it tends to throw off extra oil through the felt filter in the reservoir cap. I do not have the original instructions for this jack, so I do not know exactly what the recommended fill level is for it. From what I have read, some jacks are to be filled to the bottom of the hole while the floor jack is level on a floor. Others vary between just covering the cylinder with oil to slightly below the fill hole. At first I thought I might have a leak, but it was just the jack throwing off extra oil. This extra oil may also be due to the jack evacuating air trapped inside the jack. There are bubbles in the oil vented. One source suggested raising the jack fully and lowering it slowly twenty or so times to remove all air that might be trapped inside the jack's passageways. Check the oil level to keep it at the desired level.