Currently Being ModeratedJun 23, 2012 7:56 PM (in response to Banktank)
As far as iTunes is concerned, it is one way transfer from iTunes to iPod and not the other way except for ratings, playcounts, OTG playlist and metadata, but never songs or video.
Your 2nd advisor, is quite correct, if you put iTunes on "Auto-Sync", it will earse everthing on the iPod and replced it with the library content, so if you have an empty library, the results will be an empty iPod.
Sack the first advisor, the music in the iPod will never resync back to the Mac, you need some 3rd party software, refer to this earlier post.
Just choose from any of the huge list.