fossil oil vs crude oil

fossil oil and crude oil both are nouns.

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

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

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

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