Subscribe to The Chicago Manual of Style Online
Usage and Grammar
Q. When somebody (especially a superior who wants to get work done through you) means, “You must/should/have to do this,” is it correct for her to say, “You need to do this”? You need to answer that; otherwise I am not likely to get sleep. Thanks!
A. It’s certainly correct grammatically. CMOS is silent on whether it’s gracious or effective.