If you gun has been stored for months with either oil or any kind of lube in it, flush it with alcohol. Just plug the vent, or nipple, and pour it in. Slosh it around, to get at all the grease and oil, and then pour it out. You will be surprised at how much oil and grease will come out. Then pour in more alcohol, and repeat it, until your alcohol comes out clear. Pull the vent or nipple plugs, and pour some more alcohol down the barrel and let it flow out the vent or nipple. That will clear the oil and grease from the flash channel, nipple, or vent. You don't need a lot of alcohol for this last process.
I suggest that if the gun is percussion, you remove the nipple and blow through it. Then look through it at a light bulb. If you don't see light, get a nipple pick( wire) and run it through to remove whatever is blocking the light. Put a light drop of oil on the threads of the nipple. Use a pipe cleaner to dry and clean the flashchannel from the nipple to the chamber. Then use a cleaning patch with alcohol on it to run down the barrel, and get any remaining grease out of the breechplug. If you have a reduced size powder chamber, as is common on so many of the hawken or plains style guns these days, use a small bore brush with a patch around it to get into that chamber and clean it dry! Don't skimp on patches. You want that last one to come out as white as it goes in.
Only then are you assured that the flash channel, chamber, and bore are going to be free of the oil or lube you put in the gun when you put it away many weeks, or months ago.
When its cold outside, the relative humidity is very low. Taking a gun into your garage or leaving it in your truck, or on the porch of your hunting cabin is not going to cause a problem with the load. Just plug the touch hole, and put some kind of cap on the muzzle to keep rain water and snow OUT!PUt the hammer of a percussion gun down on a soft piece of leather you put on top of the nipple after removing the cap.
There was a suggestion made by a member here a few weeks ago that makes good sense, and is worth trying if you hunt with a flintlock. He puts a alcohol soaked cloth patch in his flashpan, with part of it extending over the barrel, and touch hole, When he closes the frizzen , the part sticking up acts like a candle's wick, and pulls moisture out of the barrel chamber through the touch hole, while also keeping mosture out of his pan.
I have not tried this yet, but I am going to do so. It might prove to be a good thing to do when walking out to a stand in the night, before its legal to load your gun( priming is considered loading in Illinois), so that you don't have to deal with plugging the vent in the night, or checking it to see its still in place in the dark. On rainy or misty warm days, it might also help keep both the powder charge dry, and the pan ready for priming when you hear something coming.
I know from experience that alcohol wiped on my flashpan dissipates, or evaporates, within seconds of removing the patch with the alcohol. In good light, you can watch it happen.
