Any advice? I'm looking for a bag to sleep in my car. I'm thinking 0 degree but I'm not sure. What is the difference between synthetic and down? Does it matter between left and right zip? Price is an issue but I also don't want to buy a cheap brand if it means freezing my a$$ off on a cold night. Any special options I should look for. I don't care about weight or how small it packs. Priorities are warmth and price.