crude oil vs liquefied petroleum gas

crude oil

noun
  • Unrefined oil; as it is found underground, consisting mainly of hydrocarbons. 

liquefied petroleum gas

noun
  • A mixture of propane and butane with traces of some other gases, which is kept in a liquid state at high pressure in metal bottles, and used as a source of gas for cooking, heating and as a fuel in internal combustion engines in vehicles. 

How often have the words crude oil and liquefied petroleum gas occurred in a corpus of books? (source: Google Ngram Viewer )