Possible Clues:

• Permian Basin sight
• Wildcat, e.g.
• Source of petroleum
• Base of rig — lubricate generously
• Derrick's site
• Result of boring maintenance instruction?
• After circular letter, the Italian rightly supplies source of fuel
• Olive, for example -- fine as source of fuel
• Time off work thoroughly boring in the main?
• Shaft beneath a nodding donkey?