Tested on Synology Rack Station RS2414rp+ with firmware DSM 5.1-5022 Update 4 confirmed on DSM 6.0.2-8451 Update 1.
A Synology NAS:
Does not support the use of CHAP on dynamic discovery.
It does support the use of CHAP on statically configured targets.
Only supports CHAP passwords between 12 and 16 characters long.
Sources: