petroleum vs rock oil

petroleum and rock oil both are nouns.

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

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

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

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