I use slin pins for every injection. I preload a month's worth of pins at a time while I'm watching tv, so I don't notice the time going by. Actual injections take under 1 minute no matter how thick the oil is. I've had no problems doing it this way for the last 6 years.
I started with 31 gauge, but now I use 28 g pins for everything. 1 ml is the max injection volume. I don't mind doing more frequent injections if needed, and I don't do higher dose cycles anyway.