Synonyms for So long:

n.

adieu (noun)
goodbye, adios.
au revoir (noun)
arrivederci, bye, sayonara, good day.
farewell (noun)
adieu, cheerio, bye-bye.
goodbye (noun)
au revoir.
so long (noun)
arrivederci, good-by, bye-bye, Auf Wiedersehen, au revoir, good day, adios, cheerio, goodbye, bye, sayonara, Good-bye, goodby, adieu.

Other synonyms:

auf wiedersehen
Auf Wiedersehen.
good-bye
Good-bye.