Swab the top of your vials with an alcohol wipe and no need to swap needles in between drawing from the vials. Other then that you're good. I personally get rid of all the air from the syringe, I like to squirt a little bit out while holding the needle up so that the oil runs down the needle, it helps to slide it in when I'm pinning (note: do not do this if tren is in the barrel).