Obviously, the more viscous the oil is, the easier it will flow, so in regards to what BBB was saying how it's a pain in the balls to get the air bubble to float to the needle, heat the oil up first.
Once I 'inject' the oil from the 3ml syringe into the slin pin, I stick the slin pin plunger back into the barrel at a very slight angle (and obviously, it'll only slide in about 1-2mm with it at an angle) but I feel that this allows a tiny bit more air out as you create a seal with the plunger. Once it's in a tiny bit (and the plunger has created a seal) hold the syringe horizontal and slowly tilt it so that the needle points up and wait for the oil to fall, then push the rest of the air out.
That's the way I do it and it's worked so far. A few more times worth of practice and you'll likely be able to do it without even getting any oil on the needle, let alone losing gear.
As for aspirating, if you're one of the people who do aspirate, then yeah go ahead and aspirate with a slin pin, too.
As for delt injections:
Say you want to shoot into your left delt:
Kneel in front of your bathroom sink/counter and rest your left arm on it so that it's resting 'across' your body (but on the counter)...
Then, take your right hand (with the syringe) and drape it over your left arm... your hand should be pretty close to your delt at this point, and in a comfortable position to inject.
I hope I explained that right, lol.
And aspirating with one hand is easy, especially with a slin pin... hold the barell between your thumb and ring finger, then take your pointer finger and middle finger and use those to pull the plunger up.