Glutes and the outside head of quads with a 25 or 27 gauge 1' or 1.5' needle is pretty much painless.
Here are some tips for reducing PIP:
1. Heat up the oil prior to injecting. Run the vial under hot water for 1-2 minutes. Heating up the oil will bring it closer to body temperature and make the oil disperse from the muscle into the blood stream much easier.
2. Gently massage the injection site for 1-2 minutes after the injection. It'll help disperse the oil into the blood stream.
3. I am a big advocate of training the muscle you injected into that day. It'll help disperse the oil from the muscle into the blood stream. In other words, try to time your glute injection on the day you train legs/glutes.