As far as the Lexus is concerned that's what the manufacturer recommends, and I think it takes into account the engines being manufactured with such tight tolerances, improvements in oil, and the overall engine design and fuel mileage considerations. I think the older strategies regarding oil (lighter in winter, heavier in summer, etc.) do not apply for the Lexus engines ?
Anyway, as far as Alfas are concerned, the European ACEA spec not being mentioned, is rather worrying..
It's not the fact that you use two different types of oil for summer/winter, it's the best type of oil for all year round protection of your engine.
The first number in the oil relates to how thin it is when the engine is cold (the lower the number, the thinner it is when cold) so in cold climates it's important to have a thin oil to protect the engine at low temperature.
The second number of the oil relates to how much it protects a hot engine, the higher the number the higher the temp it can protect, BUT, oil is also thinner at hotter temps so if you put in a 10-60 instead of the recommended 10-40, at high temps the oil may leak.
So.... it doesn't matter if you have a 0-40 instead of a 10-40 but it could if you use a 10-60 instead of a 10-40, but saying that, in order to make the oil thinner when cold, they put in loads of chemicals, which do break down a lot quicker than the oil, so the bigger the difference between the first number and second number on the oil, the more of these chemicals have been added.
Hence, if you live in a hot climate all year round, you would be better using a stable oil to protect the engine when hot (one with less chemicals inside it, say a 15 or 20-40).