tanker vs oil tanker

tanker and oil tanker both are nouns.

Word	Noun	Adjective	Verb	Adverb
tanker	Yes	No	No	No
oil tanker	Yes	No	No	No

As nouns, tanker and oil tanker are synonyms defined as:

Other synonyms of tanker include oiler, tank ship.

tanker (noun)	oil tanker (noun)
a cargo ship designed to carry crude oil in bulk	a cargo ship designed to carry crude oil in bulk
a motor vehicle designed to carry liquefied loads, dry bulk cargo or gases on roads.
a soldier who drives a tank