Pen needle length

I was wondering if there is any common wisdom, or personal observations regarding insulin absorbtion rates and effectiveness using different length pen needles? I'm very thin, and have been using 5 or 6 mm needles-- anything longer than that would likely be into muscle on most sites. Even with the tiny difference between 4 and 5 mm I seem to notice some differences, but I can't pin down exactly what the different patterns are-- comments from other users would be welcome and might help me figure it out.

Thanks,
Sam

I prefer to use the 8mm pen needles. Coming from the good old syringe times this gives me much more flexibility. Instead of injecting in 90° I prefer to use 45°. This means I will not inject deeper but the injection canal will be longer. To me this will make it much more likely that nothing will be leaking out afterwards. I am sensitive to insulin and use a half unit pen. Even small drops of leakage can have a great impact on my glucose control.