jesse,
Dr T's up in Toronto & I'm north of Denver, & we're both giving M1 15W-50 a try this winter. (Al, are you doing this too?) I've already had a few tests in the teens this fall & haven't had any problems. (My engine's a 2.0L I4; not sure about Dr T's).
If you're really worried about it, ZR2RANDO routinely mixes grades without any problem, so you might consider MolaKule's suggestion & top up with M1 0W-30 for the winter.