If your thinking these two steal the oil that people pump from the ground, think again. These two drain the oil that is in a person's SKIN! The tank they carry is what they use for this, they throw the kid in and wash the oil out of their skin, which is stored in the drum.
This was the end idea for this demon; originally this demon was to look like a skeleton draped in a red cloth sporting a birds head. This early version of the Oil-Taker demon would have flown over Demon City, then when he found a kid would wrap them in the dry cloth of his red cape, draining them of their oil.
It is funny, however, that these demons are twins, and manage to capture Inmu and Yanku who are the only twin kids in the show.