So I am going on a 6 days wildness hike. I have a down sleep bag that can comfortably cover up to 3 degrees if I have thermos on. I am considering getting a sleeping bag liner. Main concerns are:
Can someone give some recommendations on how to select a small, ultralight sleep bag liner that can improve the the warmth in the sleeping bag as much as possible?
For your three criteria the one that jumps out for me is a silk liner. It's warmth to bulk ratio is going to be the best. You'll prob get a couple of degrees (5C at most) for this.
That said, A sleeping bag liner is mainly to keep your sleeping bag clean, not particularly add warmth, though they do do this also. A better approach to add warmth when in a bag is to wear clothes. I also find this more comfortable as you can get in a right tangle in a bag liner in a bag.
I've even slept in down jackets, etc. when needed. This also keeps your sleeping bag clean as well!
Vapor barrier liner gives about 10 F. You wake up damp and they are not real comfortable but they work. Not only does it hold some heat in it prevents moisture from building up in the bag. Especially over multiple cold nights a bag will build up condensation. A buddy of mine used one year round for ultralight.
The basic criterion should be keeping the sleeping bag clean, and allowing moisture to wick away from the persons skin. If it adds to the warmth, its win-win situation.
First and far most obvious thing I'd consider is the shape of my sleeping bag: whether it is a Mummy-shaped or a regular rectangular one.
Considering that your sleeping bag is going to keep you warm enough, I'd recommend using a Synthetic liner, such as CoolMax. You'll find these ones slightly cheaper than Silk-made ones, though I do understand that you didn't consider cost as one of the criteria.
I've used one such liner just once, and it was great.
If you're looking for a liner to add some serious warmth (10-15 degrees Fahrenheit), look into the Thermalite Reactor series. They weigh generally in the 8oz range.
I would go with a silk liner.
Be sure your thermals and socks are polyesther or poly-cotton blend. They should be no more than 30% cotton.
Also, take caution with that down bag. On a 6-day hike, if it gets wet, you won't be able to dry it in time for the next night you'll need it. Down compresses well, and is unbeatable for warmth - when dry.
It is up to you to take that risk, I'm just throwing that out there for you. I've seen people pack two bags, one down, the other poly - and both meant for warmer weather, thus allowing both to pack down more easily, and yet together offer better insulation. That's too complicated for my blood, but some people like that arrangement.
You mentioned the temps can range from 9C to -3C, which means, you could be crossing the melting point. When temps stay really low, your risk of getting wet is lessened; when temps are really high, your chances increase - but then you don't need the down. When you are at the melting point thresholds, snow turns to wet, whereas if it was really cold, snow just blows off.
