I've been seen alot of bros complaining about the pain after the injection. I have come up with a theory and wanted to shre my experience with my bros. By no means I am sayin this would work for everyone, but it's worth a shot.
I usually do my injections in the mornings. I believe muscle relaxation is very important in painless injection. I also sleep on a electric blanket the night before to get my muscle nice and hot. And when I get up in the morning I stick myself.
After that, I usually go for decent cardio workout. It's like your are massaging the muscle for long time. I usually do this for about 45 mins. Then hit the steam room for aboput 15 minutes.
I don't get any injection pains other than little discomfort the next day. I havent try this with sus but other tests and oils seems to be okay. It's just my .02 and I hope this is helpful to someone. Like I stated earlier, it is by no means "cure all" for everyone, just my experience in helping with the injection pain.