I suppose that everything being equal you would want to use a lower viscosity lubricant in cold weather. However this presupposes that there is a problem in the first place. I do wonder about the fact that the aluminum upper receiver has a much higher coefficient of expansion than the Bolt carrier. It would seem that the upper receiver is going to contract more than the bolt carrier as the temperature goes down, so the bolt carrier will get tighter in the receiver. But again, that this could be a problem presupposes that the bolt carrier is as tight as it can be when at warmer temperatures. Try sticking your gun in the freezer and see if anything gets really tight, or the oil you use gets really viscous. If you have either problem then yo need to either change lubricant or slightly sand down the bolt carrier or inside of the upper receiver.