Synthetic 15w40s almost pass the 10W winter grade if not for the cold cranking test.Synthetic 15w-40 is hard to find in my area. 0w-40 and 5w-40 synthetic seem to be the most popular synthetic CK-4 offerings.
Have a look at Petro-Canada's 10W40 Duron compared to 15w40 Delo synthetic.
They are almost the same except for the CCS.