I would have thought the manufacturers could have put the supposed weight of the unit on the packaging as an extra safety check.......would be handy to know, then we could check before use. .....as well as later in its life to make sure its still got the required amount of oil inside.

if your using one I would make sure that it is protected when in transit to avoid any damage. Keep it in the box or wrap a towel around it and don't store things on top of it. If you see any oil like Aaron has don't use it.