fossil oil vs petroleum

fossil oil and petroleum both are nouns.

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

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

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

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