It is not the ester. (Directly)..it is the very high amount of Benzyl Alcohol used to make propionate ester soluable and not crash. Theoretically, if you were to take the BA out of the situation ...it would not hurt. (minus the fact your jabbin' a hunk of metal in your ass) As far as "brand", goes...some manufactures may use more BA then others in there SOPs. This is usually true with Under-ground stuff and older Mexican like tornel or brovel...they use VERY high amounts of BA in there mix.
The guys who were "baking" as suggested above and still had issues with the pain...this simply means your body still reacted to the Benzyl Alcohol...it didn't all turn into a vapor for some reason. I also would not recommend flashing off the BA all at once, unless you plan on using it all at once..(That is one big ass injection..meanig the entire 10ml vial.)
If anyone thinks I am full of shit, please prove me wrong, go ahead and blast 1ml of strait Benzyl Alcohol in your lateral side of your quad and see how fun it is to walk up the stairs the next few days.
You just need to get the BA out of the prop beforew you inject. The easiest way is flash-burn it just prior to injection. I will try to find the flash point of BA and post it for you guys...I know its pretty low.