fossil fuel vs oil

fossil fuel and oil both are nouns.

Word	Noun	Adjective	Verb	Adverb
fossil fuel	Yes	No	No	No
oil	Yes	No	Yes	No

As nouns, oil is a hyponym of fossil fuel; that is, oil is a word with a more specific, narrower meaning than fossil fuel:

fossil fuel: fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content
oil: a dark oil consisting mainly of hydrocarbons

Other hyponyms of fossil fuel include coal, gas, natural gas, crude, crude oil, fossil oil, petroleum, rock oil.

fossil fuel (noun)	oil (noun)
fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content	oil paint containing pigment that is used by an artist
	any of a group of liquid edible fats that are obtained from plants
	a slippery or viscous liquid or liquefiable substance not miscible with water
	a dark oil consisting mainly of hydrocarbons

fossil fuel (verb)	oil (verb)
	administer an oil or ointment to; often in a religious ceremony of blessing
	cover with oil, as if by rubbing