You are injecting into a vessel... You need to insert needle, pull back on the plunger a tiny bit... if you see blood you are in a vessel. At that point either pull back neddle a little bit and try again. if no blood ... then good. BTW I do this for a living (RN).
i hate to say this because thats what you do for a job, but that is not necessarily true...
i aspirate every injection, and even then sometimes some blood will come out... sometimes just a bit, sometimes a lot... what it is, is when inserting the needle you are poking through or nicking a vein, so when you pull out it follows...
if you were to inject into a vein there would be more to it, light headedness, possibly passing out kind of stuff...
as for the leaking oil, that happens... you may not be injecting deep enough, what size pin are you using? but i also have this happen sometimes...
to prevent it do a search on 'z track injections' or you can even draw a little (like .2cc) air into the barrel and when you inject the oil, the air will follow and 'pack' the oil in... that is what i do, and very seldom do i have oil leak as much as i used to...
the more you do this, the better you will get at it, it takes time to learn...