It's been asked in the past and the overwhelming response was Boge
Slim 510 should use the standard resistance Boge 510 cartomizers. You could use the 2.5 ohm if you can find them but they would use up your battery real fast and small chance they could weaken your battery. You cannot use the 2.0 ohm hotter ones - those are for fat battery 510's like Ego an Riva 510. Other types of attys down in the mid-1's are for 510-threaded mods with removable industrial batteries.
I recommend you try the Boge ones before experimenting with anything more exotic.
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How I fill Vapor4Life KR8 cartos and Boge 510 cartos
Pop out the soft cap with an unbent paperclip in the mouth hole, very careful not to catch stuffing or the wire loop.
Shake and open the eliquid
Hold a bit of tissue on the threaded end hole, tilt the carto, and add 18 drops per ml of capacity into the stuffing (Boges and most KR8 cartos are about 1ml).
Wait 15 seconds or so then see how many more drops the carto stuffing will soak in easily.
Clean up the threaded end, tilt open end down into tissue, puff into threaded end to clear air channel (Boges and the KR8 cartos I use have a defined center air channel and I want to see light through that)
Sop up any loose juice above the stuffing
Cap and let it settle - I lay it on different sides for 15-20 minutes to make sure every bit of stuffing gets damp so I won't singe the carto. if you use thicker juice than me (I use 20% Vg or less) then you may need to wait much longer.
While vaping, if the vapor gets a bit light or a bit dry or the carto is getting hotter than normal, I take the carto off the battery and add eliquid. I sometimes use a plastic or delrin drip tip as a mouthpiece on Boge and KR8 cartos (not to drip into since the juice would head for the central air channel)
And if you have auto batteries, be very careful with cartomizers. If they leak a bit, they will do it right over where an auto battery can be damaged or killed - over the hole in the auto battery's threaded end.