Why would you run a heavier oil in the winter? I would stick with the 5W-20 like the oil cap says. Using too heavy (and too light) of an oil at any time of the year will lead to higher wear rates inside your engine. I know there's a lot of "common sense" about what people think about motor oil, but the manufacturer specifies it for a reason and unless you've got proof that a heavier oil is better for the engine, you've got no real reason to use otherwise. Stick with the 5W-20
+1 on the 5w-20. I typed a big long response last night, but when I tried posting it, the damn computer shit the bed. So I'll just say, 5w-20 is suitable for that engine in all weather conditions, and you should use it, and only it.
5w-20 turns in perfectly fine used oil analysis in engines designed for it. There is a website called www.bobistheoilguy.com where people (like me ) get UOA's and post the results. If it calls for 5w-20 it will be fine with 5w-20. I wasn't a believer either until I got some UOA's, after that I could sleep at night.
My wife has a 3.0 Taurus that calls for 5w-20, Motorcraft oil got just as good a UOA as Mobil 1 for a 5000 mile interval.
Bottom-line best UOA I ever got was with Pennzoil Platinum synthetic which is a few bucks cheaper than Mobil 1.
These days I pretty much use Motorcraft oil in everything. Hard to beat the quality for the price. It is made by Conoco.