I know most of you know this, but for newbies: a 23 1.5" means a 23 gauge needle that's 1.5 inches long.
The confusing thing is that the HIGHER the gauge number, the thinner the needle. So a 21 is pretty fat, while a 30 is one of those very-thin insulin needles.
Most people use 23s for glute injections, but if you can find 25s (and you can get the oil through such a narrow needle) it should be more comfortable.