I've used M1 15W-50 for about ten years now, and my father about 7-8 in our respective, heavy, big V8 vehicles. Lows in our part of the country rarely hit below 20F (geeze, so I readjsut the choke), but hot weather running is another story altogether. (20W-50 is on the factory list for both of our vehicles; a 1971 Chrysler 383 and a 1987 454 Suburban).
I'm not about to tell you that I am averse to other picks, but in our case[s] this oil reduced consumption significantly and became more predictable to boot.
I lost a water pump one night and continued driving another 12 miles before being able to stop (though I was barely moving at the end; the preignition rattle was unreal). Had to wait 2-hours sitting outside 7-11 just to let it cool off enough to hose some water in.
Replaced pump, refilled coolant, changed oil and filter. Drove another 10k or so before pulling heads to decarbon, replace seals, etc. 26-year old oem head gaskets were, amazingly, just fine. Have gone another 35k since then.
May not be the right choice for you, but has served very well for us.
Were it me, I'd carefully consult the factory recommendations, and hire Terry Dyson to guide me through a couple of analyses. Here's some of my experience on one vehicle:
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=3;t=000646
(Link shows first analysis.)
(Current oil fill is Redline 10W-30 with a Donaldson P169071.)
Take advantage of what this site offers, and spend a few bucks to get numbers you can base your judgement upon. That makes it worthwhile for me.
After this vehicle, then I have a couple more to play with. Once oil/filter/interval is reasonably established, then I can see to the other concerns of long-life. No more hit-and-miss. Just the occasional analysis to see if trend is holding and that problems aren't cropping up.