fossil oil vs fossil fuel

fossil oil and fossil fuel both are nouns.

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

As nouns, fossil fuel is a hypernym of fossil oil; that is, fossil fuel is a word with a broader meaning than fossil oil:

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

Other hypernyms of fossil oil include oil.

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