residual oil vs fossil oil

residual oil and fossil oil both are nouns.

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

As nouns, fossil oil is a hypernym of residual oil; that is, fossil oil is a word with a broader meaning than residual oil:

Other hypernyms of residual oil include crude, crude oil, oil, petroleum, rock oil.

residual oil (noun)	fossil oil (noun)
oil products that remain after petroleum has been distilled	a dark oil consisting mainly of hydrocarbons