The benefits of using the correct 0W grade motor oil for your vehicle are faster flow at start-up to reach critical engine parts, faster engine warm up and delivers better fuel economy as compared to higher viscosity grade oils.
OK, so instead of regurgitating oil company marketing, put on your critical thinking hat for a minute.
1. Oil pumps are positive displacement, which means that as long as the pump isn't on the relief, the same volume of oil is being forced through the engine. So "flow" is the same provided those caveats are observed.
2. The only place #1 fails to address is cylinder wall lubrication, which is achieved by the shearing action of the rod on the rod journal, which forces out a spray of oil that lubricates the bores. Some engines also have piston cooling jets that spray on the bottom of the pistons to aide in heat removal (my 6.4L has these). Both of these mechanisms work better with thinner oil, and since all oil thickens when it cools, the closer to "hot" viscosity the oil is, the more effective these are going to be. In application, this has very little impact on longevity however.
3. It does not lead to faster engine warm-up as we've already discussed. An engine's generation of heat is directly tied to how much fuel it consumes. Remember, engines are horribly inefficient at converting chemical energy into mechanical energy, the biggest share of that chemical energy ends up as heat energy. Ergo, the more load that's on the engine, the more fuel it is going to use, the more fuel it uses, the more heat it is going to produce and the more heat it produces the faster it is going to warm up. As I already mentioned, you also have the shearing effect of the bearings on the oil itself and heavier oil generates more friction, which in turn means more heat, as well as more drag, which means more fuel. Follow?
4. Fuel economy. Absolutely. As I already noted, the purpose of using thinner lubricants is to primarily improve fuel economy during the period where the engine is least efficient, and that's during warm-up. The closer the oil is to operating viscosity, the less drag is placed on internal parts, the easier the oil pumps, and the less fuel the engine has to use to provide the same power. While these are all small fractional gains, they do add up to a measurable impact on fuel economy.
As I already said, OEM's have been working to decrease warm-up time of both oil and coolant and they've been doing this by:
1. The fitment of electrical fans in place of mechanical fans
2. The fitment of coolant/oil heat exchangers. Coolant comes up to temperature MUCH faster than oil does (where does the majority of that aforementioned heat energy go? Right into the coolant) so these devices use the coolant to heat the oil, rapidly reducing viscosity, drag, and the impact on fuel economy.
3. The fitment of grille shutters. This is probably the most recent "tweak". Block off airflow through the radiator, which causes the coolant to come up to temperature faster. Couple that with a coolant/oil heat exchanger and you significantly decrease the time required to get both fluids up to operating temperature.
Also, keep in mind, the number in front of the SAE grade, so in this case, the 0W-xx is the WINTER rating of the lubricant. A 0w-40 is significantly heavier than a 0w-20 and a 0w-20 with a low VI can be heavier at most temperatures than a 5w-20! However, the beauty of the 0w-xx Winter rating is that it works anywhere from Texas to Alaska, which allows these oils to be "universal", which is why they've become so popular in recent years.
