From the Oregon Chain website, they only have one chain for the stock Echo bar. You can go to a thin kerf chain if you get the thin kerf bar. My CS-4400 came with the 18" bar and chain also. I bought a 20" bar and chain for some larger cutting I did, and just today bought a 16" Oregon bar and chain combo for general use. The Combo was only about $9 more than just an 18" bar (when I bought the 20" combo I was pleasantly surprised to find that it was also quite reasonable).
Here is the Oregon replacement chain for the stock 18" bar:
ECHO CS-4400 18" .325" .050
Most Oregon bars and replaceable sprocket-nose kits DO NOT accept the Guard Mate tip guards supplied as original equipment on Echo chainsaws. When used without tip guards, there is an increased risk of kickback that could result in serious injury to the saw operator or bystanders. When replacing bars or sprocket noses on Echo saws, Oregon recommends that bars and noses with Guard Mate holes be used and that the bar-nose-tip guard be reinstalled. Echo saws without bar-nose-tip guards should be used only by those with experience and specialized training for dealing with kickback.
.325 pitch Micro-Lite bar and chain MUST be used together
33SL072G PRO-GUARD™ (33/34/35SL) 72 .325" .050 7