Kschachn: As noted, a topic that has been beat to death multiple times and in a variety of ways. Left, right, up, down and sideways. Also very related to your previous thread
Castrol Australia website different from USA.
Of course all of this was already answered by multiple individuals in multiple ways, but here we go again:
No engine is "designed around" a specific grade of oil. It can be designed as to allow operation with certain thinner grades without incurring excessive wear, but this does not preclude the use of a higher grade. An engine is not damaged by an oil with a higher HT/HS but it can be damaged by an oil where the film thickness is too low.
What automakers do now (both here in the US and nearly worldwide) is to chase after increasingly stringent CO2 emission levels. CAFE here in the US used to be based on fuel economy but it changed to align with UK and EU regulations about CO2. Automakers have various strategies to accomplish this and one of the strategies is to utilize design changes that permit the use of thinner and thinner lubricants within the vehicle. This applies to the transmission and elsewhere not just engine oil.
It has less and less to do with operating environment (ambient temperature) than it did years ago. One can see this in the widespread use of oil coolers and other attachments, so the oil can be kept at a temperature which is low enough as maintain an acceptable MOFT that won't damage the engine. Correspondingly the recommendations for oil grade (which despite gross misunderstandings on this are still only recommendations) are now based entirely on regulatory requirements rather than engineering.
Automakers are spending boatloads of money to meet CO2 regulations. Engine oil is just one of the ways this money is being spent.