petroleum vs crude oil

petroleum and crude oil both are nouns.

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

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

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

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