used an 18g at the start and let me tell you something. fuk that shit
I don't know how many times i've had the 18g chip off a small big of the rubber stopper on the inside of the vial down into the oil and had it drawn into the syringe just about to pin when I spotted it
needless to say I now use a 23g to draw, yea it takes a little longer but hey no worries about making shit of the top of the vial and or injecting a piece of rubber into your ass
although I couldn't imagine it fitting through a smaller needle after you switch them before pinning... still though