When I look at an 18ga my mind invariably starts thinking about stopper coring. Then I of course think about glute coring. Then I smack myself around and stop being a bitch.
Seriously though, I'm a huge fan of injecting warmed oil with 1.5" 27ga needles. As long as the oil is warmed, it'll go. 27ga looks like something one would reasonably use to inject substances (unlike the 18ga). Being 1.5" means you get it deep IM (That's What She Said).
I've started doing a bit of Test Suspension lately. The 27ga won't work at all. I'm sure an 18ga would, but again, Muscle Coring isn't something I want to know anything about.
The "sweet spot" for Test Susp from my personal Scientific Method of just fucking trying it, has 21/22ga as the way to go for Test Susp. With some effort you can make a 25ga work, but a 22ga should work for anything you're trying out.
Be Safe,
GPB