I tried using one of those silk liners in hopes of bringing a 30 degree bag (Marmot Hydrogen, excellent bag) down a few degrees more. At least for me, it didn't work--I'm a cold sleeper. Plus, by morning I was so tangled up in the liner that it took me a couple of minutes to get out of my sleeping bag!

Unless your bag fits quite loosely, putting a fleece liner inside may be enough to compress the insulation at your wide points. That will make the bag colder than if you didn't have the liner!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey