I would recommonmend down cause its lighter and warmer then sytetics.
several brands have expedition sleeping bags of good quality..
Here some of the most extreme of some good quality brands
Western Mountaineering
Bison SDL
Down, _40 F
Marmot
CWM
-40°F • -40°C
Mountian hardware
Seti EX SL
-35 F / -37 C
North Face
Inferno Endurance
-40°F/-40°C
Just som examples..most brands have a bag like -30 or so witch is cheaper offcourse...but spending that amouth of money why risk it..MtVinson is cold

There are much more offcourse..every serieus bag manufacturer has 1...maybe not always in there catalogue but they do make em (small market)
Just type expedition sleeping bags on google and you wil find lots.