I've got an original Lava Lamp brand and I still have the box and instructions. Unfortunately, they don't give any specs about what it contains. They also warn against adding any fluids, etc. They mention the empty space at the top of the glass vessel to allow for expansion. On mine, it is about 3/8" below the bottom of the top plastic cap. I suppose if you can confirm what the fluid is, you can add some, just be sure to leave enough space at the top for expansion.