Sustanon 300??? Hmmm I am no expert, but I have heard something about it, but not sure if it is legit, nor what it contains. I have some Sus 250, but it looks nothing like what you describe. You sure it isn't T 200 or T 400???
Then again, I am no expert on this field, but I am curious. If you say the shot burned, my sustanon does not burn, but hurts like a biotch the next day and for about another 4 days after that. But as far as injecting it I feel nothing... it actually feels kinda good injecting (course I am a sick mofo and like needles.. sooo I dont' count).