Nothing man has ever created is as warm as the items mother nature has created. Down, wool, and real fur, are the warmest.
Try sleeping at 35 to 65 below in a synthetic sleeping bag, ain't going to happen. With real goose down, takes about 10 minutes to warm the bag up, then it's off to sleep with you.
I wear Beaver or Rabbit fur hats, Goose Down Bibs and Coat, Wool socks and thermals, with beaver or shearling mittens. I also wear a Columbia Wool Coat with a Goose Down vest over it. Very warm combination.