Here’s my thought process I can’t claim it to be the best practice but it’s what goes thru my head.
Im not pushing any air into second vial before drawing so in theory If anything does get into second or third vial it’s going to be trace amounts. If it does and even if the oil is different (which it is) its coming from another sterile vial so it won’t matter.
Now let me give you the problem with my technique. Suppose I draw from test vial and the test vial happens to be contaminated for whatever reason from the person who brewed it. I then draw from mast and tren and later find out I got an infection due to contaminated gear. All those vials are now trash. But at the same time sense I’m mixing the oil in same barrel I would have no way of telling which vial it was even if I did use different needles so they would all be trash anyways or I’d have to refilter them all.
I always draw my smallest dose first for instance test P at 30 mg a day because I need that to be accurate on syringe so that allows me to push back into vial to get accurate number. Then follows the mast and tren as long as I get those close to dose I’m cool. That’s why in my tren a experience post you will notice I always list my tren dose at 50-55 or 75-80 because I’m using same syringe there is always that small margin or error it’s hard to beat when not able to push back into vial