It is precisely this time of year, ie; Zero Weather, that causes guys to turn to the 10w-30, or 5w-40 synthetic Rotella T. (WalMart $20 gal)
That 15w-40 would be mighty, mighty stiff at my place this morning. You didn't state your location, but that HIGHLY influences one's recommendation.
Heavy oil turns to super glue at -10.