I can't find the reference right now, but we had some professional papers that show cold start wear occurs mainly in the ring band on the cylinder walls. SAE 30 straight grades was found to be the best at preventing it because it had no viscosity modifiers (VM's) and viscosity index improvers (VII's), which have no real lubricity value ...
As the oils have gotten better of late with chemistries that yield wider viscosity spreads w/o VM's and/or VII's, they are yielding results closer to the SAE 30 residual film strength on cold surfaces. So they can rightly claim to reduce cold start wear by some factor (against traditional multi-grades). And we get the benefit of less viscosity and easier starts in really cold conditions.
All engines are wet inside all the time. Residual oil films protect on initial start. Bearings are protected by pumped oil in a few seconds. But they could run longer on residual oil. Cylinder walls are oiled by sling oil off the crankshaft and the oil control rings scrape most of that off with each stroke. So they need new oil soon. But cold thick oil does not sling as well (say SAE 30 at near freezing), so this area suffers. Add to that the enriched fuel/air mixture which dilutes and washes oil off the upper cylinder walls, and you have wear.
We have all now seen modern EFI engines with 200,000 miles with the cylinder hone pattern still showing. That is proof that much better fuel control (compared to carburation) and modern oils have gone along way toward curing this issue withing the bounds of the average car or light truck. Most will rust and fail before engine wear is an issue ...
Big rigs now routinely go over a 1,000,000 miles between major overhauls. We are living in the golden age of lubricated internal combustion engines. Most all is well
However, what will happen when we move ever closer to bio-derived renewable fuels like E85 for the average drive, I do not know. Alcohol can create a whole new round of chemistries that can damage oil and engines ...