New to Air Pistols

Good afternoon. I recently purchased my first air gun (Umarex Glock 19 Gen 3) primarily for trying to keep the squirrels away from my wife’s bird feeders.

I really like the way it fits and feels in my hand and designed a “speed loader” using a plastic straw and the box from an Apple Pencil. I have and have put around 300 “rounds” through it this far. I have been putting the small drop of oil on the top of he cartridge before tightening it down and now there is, what I call “smoke”, coming out the end of the barrel with each shot. It has only shown up in the last two cartridges.

Is this a problem? If so, what is the remedy/fix?

Thanks.