Brian, I hate motor oil threads too but I hate them because there so much misconception/false information/personnal unfounded opinions/tales/hear-say. I spent over 25 years of my professional in the lube business, developping/testing/blending/comparing/analysing all kinds of lubricants for an with engine manufacturers/ aftermarket customers, car makers. I think I have a little bit of knowledge in this department.
Motor oil, whatever make/grade/type/ application needs to be replaced at prescribed interval not only because it has lost it's some lubricity, but because it is dirty and because the additive package has lost a good portion os it's properties. After 4 to 6000mi, motor oil becomes a soup of oil, water, sulfuric and other acids, ash, gasoline (diesel) gum, soot, varnish, carbon, metal particles, silicate and many other contaminants. It is the combined ill-effects of those contaminants that are harmfull to an engine.
A $25./quart super extraterrestrial synthetic oil will be as bad/contaminated as a 79c/qt oil.
Synthetic oils have properties far superior to mineral oils for certain applications (transmission/differentials/grear box/ hydraulic systems) where oil operate is in close confinement (not exposed to external contaminants) but for and engine, a fresh good grade mineral oil is by far more protective for your engine than a 6,000miles old high priced synthetic oils.
I must say oil companies have had some very successfull marketing campains to promote their higher price synthetic oils for engine.