Man, you guys must all be very vascular because I have not hit a vein yet in 5.5 years (though I'm sure there is a first time for everything). Maybe I have really dense muscle without many veins going through it--I did a quad shot for the first time this morning and though for sure I would hit something, but I didn't. I have had a tiny bit of oil come out before, but it was clear.
How much comes out and how much are you injecting? If you are injecting say 1 cc and a whole lot of blood and oil comes out then that could account for there being less pain, but if you are only getting a small amount out there must be some other reason for there being less pain. Consider what the prop is suspended in or dissolved in--blood is typically on the alkaline side so it might neutralize some of the effects of the vehicle the prop is suspended in whatever that might be. Some of you chemistry wizards jump in here...