petroleum vs fossil oil

petroleum and fossil oil both are nouns.

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

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

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

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