I think there is a tread about problems running a B/W prints on color machine aside my personal opinion that is it foolish, waist of technology and it's capability. Previous poster is correct - you will create a fuser marks (and it is not even a major problem running B/W) that will affect future printing of anything larger plus you will run-up the lifes of everything up running just BS b/w stuff and when quality sensitive color order comes in - what are you going to do?
Get a B/W printer for that or at least something that not as expensive and crucial to your operation. Get OKI CX3641 or similar, it is color but when run in B/W mode CMY drums are raised up and do not turn, produces nice b/w, takes 330 GSM, and dirt cheap.
And you'd be able to run some short run envelopes as well....