You can use a synthetic bag. The only reason I got a down bag was because I got a good deal on it and it compresses better than synthetic.
As long as you don't mind having no room to move around in, you can use a mummy bag. I prefer to have room to move my legs when I sleep so I went with a rectangular bag.
Because of my dislike of how constricting mummy bags are, I'm presently taking mine and turning it in to a UQ.
"The difference between genius and stupidity is that genius has its limits." - Albert Einstein