What's the story on that engine failure?
Yeah, a HTHS difference between 3.5 and 3.8 cP isn't huge, and HTHS of 3.5 cP is pretty good for a 5W-30 and would work fine for street driving, but as usual the higher HTHS will give a bit more protection headroom. The lower Noack is a bonus too.
It's one way to look at it, and you're basically looking at what you believe will be the highest oil temps and shooting for a KV100 that's going to keep the viscosity 10 cSt or higher. As the max oil temp goes up, then to maintain a safe level of viscosity the KV100 & HTHS viscosity will also need to go up. Running a xW-20 at say 250F max for instance would be running the MOFT on the ragged edge IMO with not much if any protection headroom.