The needle size doesn't matter, just be sure to stay in fat with the needle.
5/8 depending on the injection site can be just enough to get through the fat/skin,
but too shallow to enter muscle. The area between the two is a place
you don't want the oil to be.
Most SUBQ of test is done by TRT guys, and they are using smaller amounts
say .125 or .25ml. Even at this small volume nodules can be left behind and
cause discomfort. .5 to 1ml will leave a nodule for sure.
If you want to use a smaller needle with higher volume I suggest 27-29g 1/2 slin pins.
They leave very small holes and you can get IM in the delt or trap with ease,
you can also use them for SUBQ if that is the road you want to travel.