It is quite clear from SAE J-300 that 15W is intended for use down to -20C (+4F). I used 15W-40 regularly in a 1994 5.0 in winter temperatures here (+20F and above) with no signs of slow cranking, piston slapping, lifter tapping or any other oil-related problem.

The only oil that miserably failed a "freezer test" was Coastal API SA non-detergent SAE 30. The streaks of wax were visible in the gel. Delo still poured, but I would not use it at 0F, since it is not rated for that.

Jim