Lots of good advice above. I agree with many of the previous posters but I might add a couple of things...
I also live in MN and know very well what the winter is like. I used to despise treadmills but then I bought one last year and had the best winter of training in years. In my opinion, the treadmill is a great way to do tempo and marathon pace runs during the winter base period.
I set mine at 1.5-2% grade. I've seen two grade/speed conversion tables, the one cited above and one in the Jack Daniels book. They are helpful but you should also remember that the speed of one particular machine might not be accurate and 10mph on one treadmill might really be faster than the same setting on another. If you go by perceived effort, the advantage of the treadmill is you can inch up to the speed/grade where you want to be and then lock into that pace.
Another note on grade: I often throw in some simulated hills when running on the thing. For example, every 10 or 15 minutes I might crank up the grade to 5 % for a minute or two just to break the monotony and work some different muscles. Makes the time go faster.
Finally, use the time on the mill to think about your form, cadence and breathing. It's much easier to analyze these things when you are running at a constant speed and slope. You can see how slight changes in posture can make you inch forward or backward on the mill, .i.e. make you go faster or slower. You could never notice these subtle differences in speed outdoors. Added bonus: it makes the time go faster.
Here's to a mild winter here in Minnesota.
Good luck.