I'd say there's no real benefit until the temp is well below -10F, and then 0W-20 only has a small advantage over 5W-20. Heck, at -10F even 15W oil is pumpable. I would agree that recommending 0W-20 could be a way for the manufacturers to get consumers to use synthetic without coming right out and using the word "synthetic" in the owners manual. Valvoline, Castrol and Pennzoil all make a synthetic blend 0W-20 but they're not commonly found in the oil aisle at places like Walmart. At one time Toyota sold their brand of 0W-20 synthetic blend but I don't know if it's still available.