Landfills process their leachate. They all leak from above (rainwater) ..and are allowed to leak from their liners. Oil would not just lay in the landfill. It would probably be pretty clean due to the "ground filtering" when it ended up in their waste water treatment plant. Depending on the system, a skimmer should remove it from the waste water stream. Some even recirculate it. I think it's a method of allowing natural venting, from heating, to deal with the rainwater that invariably finds a way in. They're all vented and even power generators with the off gassing.
Some bacteria will attach to just about anything. That's how you remove some chemical pollution where removal is just too impractical. You take a sample, identify the bacteria ..and mutate it in a lab. You then introduce it into the site and either aerate it or turn/churn it ...eventually, you have no more waste ..other than the byproduct of the bacteria.
..but (short verson) ..I dunno. Good question.