Embolism possibility
Fellas,
The concern over injecting either one's own blood back into the body in this manner or a large amount of oil into a vein is based on the theoretical possibility that you might form an embolism (blockage in the blood stream - blood clot or foreign body), leading to an cerebral aneurism and a stroke, or that it might lodge in your coronary arteries and cause a heart attack. It also go to the lungs and cause a pulmonary embolism.
If you've got lets, say 1cc of blood at one end of a syringe, you pull it out, cap and put the needle down, prep another are for injection, there is just a little amount of time when the blood could start clotting. Obviously, injecting a clot is not a "good" thing if it makes it way into yet another vein. I don't know how well the body can dissolve this, but if its not taken care of in about 1 minute (time for all the blood in your body to circulate), then it could lodge in a small vessel (see above). If its in a vein, this is much better than an artery, b/c it will travel back to the heart and possible get broken up in the turbulence in the heart.
A big glob (technical term) of oil floating would prob. be broken up in to little droplets, but the fear here (I would think) is that the droplets would encounter an artery, say a coronary artery, that is nearly occluded already, adhere to the fatty plaque, end up fully occluding the artery and , in this case, causing a heart attack. I most of us young 'uns, we don't have much to fear here, but it is known that cardiovascular disease begins in childhood and young guys have died of heart attacks...
So, all of this is pretty remote, but this, I think, is the rationale behind not injecting clotted blood or a big oil bolus directly into a vein.
-Randy