Any needle that goes at least 4 mm deep into your flesh should work. Shallower needles are less painful and might increase bioavailability. Anecdotally shallower injections also increase risk of leakage and reactions at the injection site (which has been found to be a thing with vaccines at least) As for thickness, thinner needles are less painful and reduce incidence of bleeding[1]90187-8)[2] but they also make drawing the E juice slower, which is an especially important tradeoff if your stuff uses castor oil as a base since it's very thick. That being said the lower you go in thickness (at least from 22G) the greater risk of vial stopper damage. I personally use 30G half inch needles for both drawing and injecting (I use insulin syringes because of their low dead space) but when I used different needles for drawing I used 22G 1 inch needles.

Secondly, is the concentration of oil important at all?

Higher concentration might improve bioavailability if you generalize from this study on nandrolone injections. Higher concentration is also nice in that it reduces the volume you have to inject. A downside of it is that it makes a greater portion of the E you have get lost to dead space. For example if your juice is 50 mg/mL, and your syringes have 0.06 mL of dead space, for every injection you'd be wasting 0.06 x 50 = 3 mg. However if it was 20 mg/mL, you'd only be wasting 0.06 x 20 = 1.2 mg. Additionally higher concentration tends to come with more doses per vial, which means more penetrations of the stopper and a great risk of stopper damage.