You need to use a degreaser, I was doing remediation work in the oil industry for 2 years. We used mainly Simple Green or Equinox mixed with water to decontaminate equipment with gas/oil byproducts. You should be able to buy Simple Green at Home Depot.

What I would do is let simple green and water mix sit in the barrel for 24 hours. Empty and then proceed to scrub the inside with about 2 gallons of simple green water mix. I would then pressure spray the walls with water. Then finish up with a water rinse. It should be fine after that for garden usage.