I agree you needn't be overly concerned about doing harm to your engine by leaving the 5w- 20 in for a change interval. The manufacturer's choice of viscosity is a compromise of factors chosen to balance wear throughout the engine. Although a thinner viscosity might slightly increase wear in one part of the engine, it will result in less wear in other parts. The oil performs a lot of different duties at different parts of the engine. Note that there are more differences between oils of different viscosity grades than simply the viscosity – the additive package varies as well, so the manufacturer may have chosen one oil over another for the accompanying additive package.
Because engines operate under so many varying conditions and duty cycles fuel dilution is not a given in all situations. Higher compression engines like the Eco-boost do allow more fuel to move past the piston control rings and into the crankcase, but if the engine is run long enough at operating temperature the fuel and other volatile contaminants become vapourized and burned off. Short trips and cold weather can be a culprit too. The superior fuel control that direct injection offers should actually reduce the amount of fuel dilution, as compared to more primitive carburetion for example, particularly during warm-up.
As an example, fuel contamination is below detectable amounts in the oil analysis posted above, post 64 by jrgoffin, (as measured by the flash point, more fuel in the oil=lower flash point). The viscosity did test in the “low” range, however there is no way to know for sure what it was when new – it's possible that the oil was not far from that initially.
As oil is used over time, it thickens, unless diluted, because oil, especially non-synthetic, has had viscosity improvers added to it to make it thinner. The base oil remains intact for a long time, but the additives diminish in time, so the oil thickens. Contaminants contribute to this as well. Loosely stated, the oil is thick oil with thinners added to it, so as it is used it thickens, sometimes to the consistency of molasses, as a quick 'boo at Youtube will bear witness to. Over-simplified, I know.
I am still curious about the fuel dilution though, FPO - have you seen frequent fuel dilution happening with these engines, as judged by an oil analysis? Is there a technical explanation to offer for it?