Hundreds of thousands of Australians (including myself) have used it with no issues whatsoever down to around -10C.
My Caprice (L67 3.8) specs 20W50, with 15W40 for "prolonged use in cold conditions"...I don't use either, sticking to 10W30 or 5W30 A3/B4.
In my Camira (J Car), not long after joining BITOG, I ran a winter on 25W70, just to prove that sub freezing, things still don't blow up with 20W