Good low temp performance or "borderline pumping temperature" is what separates the men from the boys, so to speak.
It may not matter too much at temps greater than 0 F, but every 10 F lower than that appears to have almost an exponential effect on starting performance and especially oil flow. This also directly effects wear rates.
The SAE has determined that with a yield viscosity >60,000 cP, oil cannot flow into the pickup tube and be drawn into the oil pump. Therefore, "dry start" in a big way. This is part of the SAE J300 test and the determination most oil companies used to calculate MRV BPT's (Mini Rotary Viscometer Borderline Pumping Temperature).
I have a 2000 GMC Sierra with the Vortec 5.3 that I ordered new. Something about the oiling design has me quite concerned about BPT: rear sump oil pan, oil pump mounted to the very front of the crank, and a looooooonnnng oil pickup tube.
When new, with the no-name "low bidder" factory 5W-30 oil, it went "knock knock" at a temp of only -10 F. I changed to Mobil 1 0W-30 and it never went "knock knock" again.
Where I live, -40 F winters are NOT uncommon. Heck, went to -42 F last week, though today it's a balmy 3 F. That's why I run Mobil 1 0W-30 in winter, though IMHO it's HTHS is a little thin. I run Mobil 1 10W-30 in summer.
I have seen cars around here snap the oil pump drive shaft trying to do a boosted start at -42 running a conventional 5W-30.
FWIW the Esso Oil Products Handbook has a caution on "expected ambient temperature" and suggests the following absolute minimum temperatures for the grades listed:
15W-40 -20 C (-4 F)
10W-30 -25 C (-13 F)
5W-30 -30 C (-22 F)
0W-30 -40 C
0W-40 -40 C
The "0W" oils are partial synthetic HD diesel engine oils. A full synthetic like Mobil Delvac 1 should be good at -45 C.