It looks like it would make a great roller. I would remove the oil ASAP and fill it with concrete. 15 gallons of oil can make an awful and expensive mess.
I plan on filling it with sand and dislodging the oil that way. I have a container big enough for it to sit in and catch the overflow.
Another suggestion made was to fill it with oil dry or kitty litter. It would dislodge most of the oil, weigh quite a bit, and soak up any oil that's in there.