The "Add 1qt of oil when refueling" message

So I've had my car for about 5 months now and just happened to get this message, but only after driving the car for about 15-20 minutes on the highway, and it also seems to clear when I turn the car off and back on. When I drive around the city it doesn't come back up at all.
The car hasn't had an oil change since I've bought it (was done at the dealership before my purchase), but one is due within the next month / 1000 miles or so. I've read things here about the oil level sensor being bad and I'm a little worried that I won't know exactly how much oil to put in. Here are my questions:
Should I be using Mobile 1 0W-40 or 5W-40? It is colder now, so should I go with thicker oil?
Should I start off by dumping a half quart in?
Thanks!

What I was thinking about doing was just adding a half quart and seeing if the car's electronic oil level sensor notices that I added a half a quart. I'm sure this thing has burned / consumed some oil off in the months I've owned it, but I just want to make sure I don't put too much in and don't put too little in where this message comes back up.
That really tells me you seriously need 8 quarts of fresh oil not just one.




