If it's full of oil it might be a leaking valve that's letting the oil slip back by as you are jacking.
I have a floor jack that comes down slow after I raise it up, because of a leaky O-ring. In our hot summers I just replace the oil with 30-50 weight so it "takes its time" to drop down - long enough to get the jack stands under the truck.
In the winter any oil will do because its almost like molasses and the jack comes down just as slow. I do the jacking before I heat up the garage.
Its not a major problem so I never bothered replacing the seals yet - I never go under without stands anyway.
So hey wodniw, when you find the filler try thicker oil and see if there's an improvement.
Hope this helps.
