I'll be removing my barrel this weekend to do some modifications, (new float tube, gas block, BUIS,....) I've read several posts that say to use moly grease on the upper threads when reinstalling barrel........does it have to be moly grease or is there a equal substitute? Also, what purpose does greasing the threads serve?

Thanks in advance

Do a search for "molydenum"; there was a thread just last week (or the weekend before) discussing this in the Build It Yourself forum.

The basic answer is "yes, you do need this stuff."  The grease acts as an anti sieze agent (also for the lower receiver extension) so you can eventually remove the barrel.  Without it, you're likely to have the barrel nut try to become permanently part of the upper.  It also helps prevent galvanic corrosion, which occurs when two dissimilar metals are in contact in certain fairly common atmospheric and field conditions.

Moly grease IS your friend.

Molly grease molly paste or plain old ngl no.2 grease hight temp from an autoparts store will work.

What you want to steer from is anything with graphite in it. a lot of thread anti-seize compounds can be as much as 50%graphite.