On the temperature ratings, check to see if the sleeping bag is EN13537 rated. Those ratings definitely aren't 100% accurate, but they are far more accurate than those provided by large manufacturers' marketing departments. If you have to buy a sleeping bag that doesn't have the EN13537 ratings, add another 10*F to the rating, unless it's from the top of the line manufacturers like Western Mountaineering or Feathered Friends.

Deciphering EN13537 Sleeping Bag Temperature Ratings

In my opinion, it's not really possible to have an "ultralight" sleeping bag with synthetic insulation. You have to go to down insulation for a 3-season bag weighing 2 lbs. or less.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey