How much oil does it take to fill a cartridge
Let me expand on what I said.
I found that some of the cartridges I had bought at my dispensary and had already smoked, were actually the refillable type. These have a rubbery cap on the end that goes into the pen. This cap can be removed, the cartridge filled, and then the cap is placed back on, very easy.
So far, on the advice of my budtender who also uses these things, I have purchased a tincture called "Blue Dragon" that is available locally and filled the pen with that. A .5g syringe of this does not quite fill the cartridge half way.
My next step is to make some of the tincture that "badkittysmiles" recommends for these.