I may be mistaken I have been once or twice in my life( yeah I am on my 3rd and last marriage)...lol
What ever it is, it is marked for 6ML and works pretty good, jut letting it sit and drip.
6 ml is the size of your syringe, right? The reservoir holds 6 ml of liquid. 18 gauge refers to the size of the needle tip, in your case 18 gauge? Just borderline. 20 g is ideal for this cartomizer.