0w30 is fine in the summer too ... and you might even get a couple of tenths of a mpg better fuel economy. That is what M1 AFE (Advanced Fuel Economy ) 0w20/30 advertises. The 0w oil is slightly thinner than a 5w oil at cooler temperatures, so in the engine warm-up phase, there is slightly less drag and pumping loss with the 0w oil. Even in summer.
But, if you want to get all technical, here goes in a very basic sense.
Any multi-grade oil is a compromise. 0w oils are built to work well at extreme cold temps ( 0*F and colder ) . They will always be synthetic, and start off with a thin base oil, often using a mix of ingredients to get that cold flow ability. And that works well when it's cold. To get the hot temperature characteristics of a 30 grade oil, they need more additives to keep the oil from thinning too much at hot temperatures. These are called viscosity index improvers ( VII ) . Almost all multigrade oils use these additives to achieve the cold - hot characteristics desired. 0w oils usually use MORE of these additives. Over time these additives can wear out, or be broken down, possibly leading to oil that is sheared down to a thinner viscosity when hot, and possibly leaving deposits in the engine.
A straight 30 grade oil has ZERO VII additives, is very shear resistant and stable, and works very well when outside temperatures stay well above freezing. If it gets cold, the straight grade oil gets extremely thick, much thicker than a multigrade oil. It works well when HOT, but might not work at all when cold.
A 5w30 oil typically has a thicker base oil and less VII additives than a 0w30 oil, so theoretically it should be less prone to shearing and be more stable. A 10w30 again has even thicker base oil and has less additives than a 5w30, and a 15w30 (yes, they exist ) will have even thicker base oil and less additives than the 10w30, so it should be again less prone to shearing and break down.
So, IDEALLY, you could run a10/15w30 in the summer, for 30 grade hot temp viscosity, but with less VII additives to potentially break down with use, and use an 0w30 in the winter, for better extreme cold flow properties, and still have that 30 grade hot temp viscosity, with the risk of the oil thinning out through shearing and breakdown.
However, most modern 0w oil are built very well, using a complex blend of different (often more expensive) groups of oils, with better engineered additives every generation. They might in practice be SLIGHTLY less stable than a 5/10w oil, but for most applications, will work just fine year round.