I guess I should qualify that statement a bit. API Group I and II base oils have been shown to roughly double in CCS viscosity for every 5°C reduction in temperature. Those containing higher VI oils, such as API Group III and IV, increase viscosity by a factor of ~1.7 for every 5°C drop in temperature.
So in short, that doubling factor roughly applies to the CCS in the appropriate temperature region.
Jelly, the actual number doesn't really matter as long as it's within cranking specs. My manual states that a 15w is good down to -20C. I used M-1 15-50 last year down to -22C..the coldest it got in Toronto.
I just found it very interesting that we argue over a few cSt one way or another on the top-end (212F) yet rarely if ever looks at that same oils viscosity when exposed to cold temperatures...turning into a gelatinous blob when your engine demands (requests) fluid lubrication can't be a good thing...