go down on vs suck

go down on is not a noun while suck is a noun.

Word	Noun	Adjective	Verb	Adverb
go down on	No	No	Yes	No
suck	Yes	No	Yes	No

As verbs, go down on and suck are synonyms defined as:

go down on and suck: provide sexual gratification through oral stimulation

Other synonyms of go down on include blow, fellate.

go down on (noun)	suck (noun)
	the act of sucking

go down on (verb)	suck (verb)
provide sexual gratification through oral stimulation	give suck to
	draw into the mouth by creating a practical vacuum in the mouth
	take in, also metaphorically
	draw something in by or as if by a vacuum
	provide sexual gratification through oral stimulation
	be inadequate or objectionable
	attract by using an inexorable force, inducement, etc.