crude oil vs fossil oil

crude oil and fossil oil both are nouns.

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

As nouns, crude oil and fossil oil are synonyms defined as:

Other synonyms of crude oil include crude, oil, petroleum, rock oil.

crude oil (noun)	fossil oil (noun)
a dark oil consisting mainly of hydrocarbons	a dark oil consisting mainly of hydrocarbons