My belief is that most engine wear (75% or so during its lifetime) occurs at warmup between the time your engine is first starting and when it hits operating temperature as the engine runs a rich fuel mixture and there is more fuel dilution, wash down of protective oil at the upper cylinder, and blow by. It’s HTHS versus the protection of getting your engine up to operating temperature quickly and preventing washing of the protective oil layer with a rich fuel mixture. I do think thin oils get up to temperature a little more quickly on colder days when I’m monitoring the oil temp. Yes, this is where the fuel savings comes from. It runs thinner while cold and seems to get to temperature a little faster. Once the engine is nice and hot, there’s minimal fuel savings. There’s also minimal wear once engine is up to temperature… unless you are tugging a boat.
I can’t say that having metal flakes in the pan can ever be good but maybe it’s not doing damage. Even when I change my oil for the third time at 1500 to 2000 miles, I still see flakes in the drained oil.