Use a 21 ( thick ) gauge to draw the oil from the vial. After that, replace the needle (remove it from the syringe), with a 25 gauge.
i.e. you use a 21 to draw the oil
25 to inject it in.
There is NO point in putting big holes in your butt and creating unneccessary scar tissue, or possible injection complication due to using a thicker then need be needle size. You have nothing to lose, and everything to gain by doing this method.