5w30 is going to flow much better at very cold temps -10, -20 and below,but lots of older vehicles used to use 10w30 and 10w40 and still start in -20, and even -30 temps. I grew up in North Dakota ..Fargo and back in 60's and 70's my dad started the big V8 F150 and our ford Galaxy many times in temps well below -20 with a few -30 temps tossed in with 10w40 in the sump. Of course he did use a battery heater and sometimes just a small shot of starting fluid , but they always started.
Now many newer vehicles of today use 0w20 which can go to even lower temps. I think that 0w20 is probably found only in synthetic or semi synthetic though. Anything colder than -30 and I am staying home.