I'm on the bandwagon that uses ~20ga to draw and a sharp 25ga to inject. With lengths 5/8 to 1.5 in depending on muscle and volume.
The needle does cause some physical trauma, but I think you're falling into a trap thinking this is THE major cause of scar tissue. It's an issue that's been under some study, but there's not a ton out there. In any event you have to include the deposit of the oil depot into your causes of scar tissue. You're forcing oil into tissue creating a sterile abscess in your muscle where oil isn't naturally located and it contributes to the build up of scar tissue. So use the smaller gauge needles for sure, but make sure to rotate rotate rotate. Give that muscle a chance to recover. And inject slow so that you aren't injecting that oil at high pressure forcing it into places it doesn't want to go. If you have to inject a larger volume, go to a larger muscle and use a longer needed to get good deep dissipation of the oil into that muscle. Anyway, may be wrong on some of the facts here, that's why I don't have an MD after my name. Just remember to rotate, as it probably isn't 100% related to the needle and injection technique although I think those are definitely factors.