The Source, formally know as radio shack.
They have this thing on the rack called "precision luber". It's some form of light weight oil with a teflon additive, rated for up to 500F, in this little pen size bottle with an application needle.
It's awesome, been using on my AR for going on 5000 rounds now. No problems.
I use grease on the barrel cam pin and bolt lugs.
I use the mentioned oil on the rest of the BCG.
I soak a bead of the oil into the gas rings and put it together.
I run a bead of the grease along the bearing surfaces of the BCG, and on the exposed side of the cam pin.
I put a drop in the gas key and smear it around.
That's about it for the upper. Wet where it needs to be, dry where it does not. My system runs awesome.
Can't say I shoot more then 200 rounds a session between cleanings. Usually only 100. Perhaps if I shot more this method would fail, but I doubt it. I keep the oil pen in my range bag in case, have never needed it.