fossil oil vs rock oil

fossil oil and rock oil both are nouns.

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

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

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

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