To get back to Karl's original question, I haven't found another jack that fits in the cubby hole, and yes you can change the hydraulic fluid in a jack, there is jack fluid for sale most places.
I wouldn't like to be changing a wheel when the jack gives out, you are not exactly remote with arms full outstretched, and kneeling down, head forward, which may take a nasty bang from the wheel arch as it lurches down and towards you. If the wheel change is o/s, it could push you back into traffic, Definately a worry.