Why make you engine work much harder at cold start up with a 15w40 oil, in 40-30-20 degree temperatures? Your starter, and especially oil pump trying to pump up that oiltaffy, cold?
Why not put a 1/2 quart of 15w40 oil in your refrigerator for 24 hours. Then pour it out, and actually want 5 quarts of this oil in your engine, during the winter? Take a good look at that 15-40 when it pours...cold...
Why would you even want to run a 15w40 oil, in a 302 V8, Chris? It's a tired, piston-slapping, beater engine, burning blue smoke out the tailpipe, correct?
You like getting poor fuel mileage as compared to getting the peak efficient gas mileage, right? Some guys simply enjoy spending more for gasoline, and stopping more often at the gas pumps, than needed. It's an odd phenomeneon, but I appears some think like you do, Chris. We see it all the time here.
Good luck. Continue to use the 15w40 oil in your 302 V8, a good Ford Motor, by the way. Your engine will love it.