I have a BullDog Lift. Somehow I figured I'd store it on it's side, well the "stuff" inside it leaked out. You always find out when you get all set to do a good detail job, right? So the lift would not go up since all the mystery fluid was gone.
There is a little rubber stopper on the main cylinder about halfway up. Turn the release counter clockwise and push the jack all the way down. Pull that rubber plug off, empty out the bad oil, refill with Hydraulic Oil (per the manual I found online) up to the hole, replace rubber plug and you're back in action! This is a Pro-Lift 6 ton jack so I was surprised to find the pdf manual online for it. Interesting to note in the doc for the jack: you should replace the Hydraulic Oil once a year.