Here is a rhetorical question for you: If viscosity helps wear protection so much (30% increase in engine longevity by going up from 5W-20 to 5W-30), why only use 5W-30, which has almost the same viscosity as 5W-20? Why not use 0W-40, 5W-40, 10W-40, 15W-40, or even 20W-50 or higher? Wouldn't that increase the engine longevity by a factor of ten or so and the engine would last perhaps ten million miles or more?
Viscosity difference between 5W-30 and 5W-20 is very small, especially if you compare a synthetic 5W-20 with a conventional 5W-30, the latter of which will permanently shear to 5W-20 viscosity anyway. (This is due to a large amount of viscosity-index-improver polymers that make up the conventional 5W-30 multigrade oil getting permanently damaged after about 1,000 miles or even less.) So, this whole 5W-20 vs. 5W-30 is almost a moot issue to begin with.
This said, there are two basic lubrication regimes in the engine: (1) Boundary lubrication (= metal-to-metal contact). This happens at valvetrain and part of the piston rings and cylinders. Higher viscosity helps very little here and antiwear additives do the most wear protection. (2) Hydrodynamic lubrication. This happens at the bearings and most of the piston rings and cylinders. Higher viscosity provides a larger minimum oil-film thickness. If your oil is particularly dirty, this could result in less wear as the dirt particles will be smaller than the oil film and can't rub onto sliding metal parts as much. This is especially crucial in diesel engines, as they generate abrasive soot particles. Also, in extreme conditions such as heavy load and low RPM combined, a larger minimum oil-film thickness could prevent the sliding metal parts from touching each other in the hydrodynamic-lubrication regime and going into the boundary-lubrication (metal-to-metal contact) regime.
On top of these considerations, type and amount of viscosity-index improvers is important -- more viscosity-index improvers could be actually better for wear protection, as they do things the oil base stock cannot do. Type of detergents being used and other various additives being used also could make a huge difference in wear protection.
Conclusion:
It's a false statement that 5W-30 provides significantly more wear protection than 5W-20. Assuming everything else is the same other than the viscosity (similar base stock and same additive package), 5W-30 may offer slightly more wear protection in certain conditions and in certain engines. However, that's about it. Note that I said assuming everything else is the same (same-quality oils).
You're much better off with finding a top-quality oil with proven wear protection -- preferably a top-quality synthetic -- than simply assuming that a cheap, conventional 5W-30 will greatly extend your engine life. In fact, I would bet that any engine would last longer with a top-quality synthetic 5W-20 than with a cheap conventional 5W-30. If an engine specifies 5W-20, buy a high-quality synthetic 0W-20 or 5W-20 (0W-20 preferred in most cases) if you really care about engine longevity. I would use a 5W-30 in such an engine only if it's using a lot of oil so that it doesn't make sense to put more expensive oil in it (feed the oil-hungry engine with the cheapest 5W-30 you can find) and also oil consumption will be a little less with 5W-30 than with 5W-20 due to slightly higher viscosity of 5W-30.
