My amateur understanding is that HTHS is the better parameter for identifying what kind of protection an oil will give you against wear and damage, especially for hot weather driving.
I know an engine in proper condition will regulate its operating temp, but this funny thing happens where I notice my highest engine oil temps always occur in the summer!
Clearly the ambient has some effect upon engine temps, there is some slop in that feedback loop.
I could be way off here and would like to be corrected by an expert if I'm wrong, but wouldn't it be desirable to make an oil with the KV100 of an XW20 and the 3.5+ HTHS of a dexos2 XW30 assuming no chemical or financial limits?
Would you be losing anything by having the oil be thinner as it circulated around the engine at "operating temp" as long as it had a good HTHS when it got heated up by friction at the metal boundaries?