Pinning

[aus.peptides]

How do I pin multiple oils at once?
I start via injecting air into mast vial then withdrawing then I move to testosterone however there is oil in the syringe so I cannot use air for a counter pressure so I withdraw test and another issue is it’s impossible for me to withdraw the exact amount of units I need with another oil already in the syringe.
I really need advice anything will help thank you.