I highly advise to always aspirate. there are some videos on youtube that explains what happens when you inject oil into a vein. the coughing is caused by your lungs expelling the oil from your blood. it can cause a pulmonary embolism but before you start freaking out it would take approxomatley 20 ml to cause death. In my opinion tren cough is a myth, there isnt any medical reason to cause it. in my experience just clipping a vein can lead to oil leaking back around the needle and entering the vein. it only takes a very little bit to cause the cough, more oil more cough.
I have also read other peoples opinion that scar tissue can cause the coughing also, not sure on there reasoning tho.
I recently had this happen 4 times in about a 5 week period, once was so bad i puked. All was from either training partner or wife giving me the shot.
always aspirate, push oil very slow, don't changed depth of needle while pushing oil, compress and completely release the plunger before pulling needle out to help clear the tip. these have helped me avoid coughing my lungs up and feeling like shit