Cave slime in the Lumbridge Swamp Caves always drop between 1 and 6 swamp tar, which is currently worth coins each. The easiest way to get to the dungeon is by teleporting to Lumbridge, going into the castle basement with a light source, and enter the swamp caves by the entrance next to Mistag. Head south and west, avoiding wall beasts, until you reach an area with three cave slime. Consult Map:Lumbridge Swamp Caves for more details. Be careful of gas in the caves, as if you have an open light source (i.e. torch, oil lamp, etc.) the gas will explode dealing high damage and extinguishing your light source. A bullseye lantern or similar is recommended.