Unless you're buying botiques where no costs are spared there's no guarantee that 10W30 has a lower NOACK or less VIIs than a 5W30 of the same product line , like Mobil1 EP 5W30 has a good slug of PAO whereas the 10W30 doesn't seem to, same thing is noted with 0w20 compared to the 5w20, since the oil you find on the shelf is mostly made to meet a level of performance while protecting profit margins it's likely that the 0w and 5w products use better quality base oils than their 10W counterparts and that a 10W30 is not necessarily any better than a 5W30 even in hotter ambient.