Related Clues:

• Contents of some barrels
• Change it regularly
• Slick stuff
• With 'of' and 51-Across, a facial moisturizer
• Nitrous oxide, e.g.
• Welcome road sign
• Gasohol, e.g.
• Hipster's great time
• Refined crude
• Source of energy
• Tank contents
• Fuel for a Zeppelin
• Natural resource
• Drilling goal
• Pipe contents, maybe