Cartomizers, and the most popular for 510 devices are Boge. A slim 510 would use the standard resistance Boge ones (2.8-3.2 ohms). These are blanks. You can find some prefilled with Chinese juice at Vaporkings.com but you can use any thinner juices in them (I use 80PG/20Vg or less VG juices and I avoid juices that drop a lot of flavoring sediment)
NOTE - If your dad has automatic batteries (no button) then they can be damaged or killed by cartomizer leaks. If a carto leaks it will do so right over the hole in the end of an automatic 510 battery. This can damage the drag switch or short and kill the battery. If you decide to buy a manual battery, get the FS510 model only if you want to use the same charger because FS510 is electrically incompatible with all other 510 brands. You cannot charge other 510's in a FS510 charger - it will fry them, and vice versa.
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How I fill a cartomizer (Vapor4Life for KR8 or Boge 510 for 510 which are all soft-cap w/ no inner seals and defined central air channel)
I use thin (20% or less VG) liquid or Vapor4Life WOW liquids, preferably clear liquids w/ no sediment
Filling a blank carto - I pop off the soft cap (unbent paperclip carefully in the mouth hole, not catching any wires or stuffing!). I hold some tissue against the threaded end hole. I tilt the carto and slowly drip 18 drops onto the inner wall of the carto so it slides down into the stuffing while I rotate the carto. Then I wait 15 seconds or so then I drip however many more drops the stuffing wants to soak up. Then I do clean-up. If it is leaking, I clean that up. Then I tilt the open end down into the tissue and puff hard a few times into the threaded end to clear any juice out of the central air channel (I want to see light through there). If it looks too wet above the stuffing, I gently touch a bit of tissue to the stuffing (otherwise that liquid will be gurgling or spitting in my mouth). Then since this was a fresh fill of a blank, I now let it settle. I lay the carto on different sides for 15-20 minutes. Then it should be ready to vape. If you use thicker juices than me, then you may need to let it settle longer.
As I am vaping - if the vapor gets a bit light or a bit dry or tastes bit off or the carto is getting unusually warm, I will take the carto off the battery, open it, add 6-8 drops or whatever it seems to want (I will either hold a bit of tissue against the threaded end hole just in case, or skimp on how many drops I add to make sure I avoid drips if I have no tissue). I check to make sure the threaded end is dry, give it a couple of puffs into the threaded end to make sure the air channel is clear, and continue to vape.
Note - it is easy for me to "open" my main cartos for adding juice because I replace the white end cap with a plastic or delrin drip tip (I get them at LiteCigUSA.net). So I can easily pull that out with my fingers when I add juice and it keeps my lips cool if the carto is hot.