Related Clues:

• Nitrous oxide, e.g.
• Gasoline additive
• Havoline competitor
• NASCAR advertiser
• Welcome road sign
• Hipster's great time
• Refined crude
• Indy sponsor
• Inits. at a gas station
• Tank contents
• Fuel for a Zeppelin
• Engine-cleaning brand
• 500 letters?
• Natural resource
• Pipe contents, maybe