You are correct on both counts what does wear out though is the additives that makes the oil a multi-viscosity and the detergents that helps to keep it clean. what happens when the additives are gone or fully contaminated is the oil becomes a single viscosity which is loosely based on a 2/3s+2/3s-1/6 rule. what that means is if say you run 15/40 like I do take 2/3 of 15 plus 2/3 of the difference between 15&40 or 10+ 16.666you get 27 rounded up then once the oil is hot you loose another 4.4 so your oil winds up being around 22 visc. for 10/40 it works out nearly the same for 20/50 you get right around 27 visc when hot when you run the 5 or 10w 30 is when you are doing damage to the engine if you let the oil get the slightest bit dirty. because that stuff rapidly goes to a straight 15 or less.
On my old ford with the 7.3 IDI I have 2 half gallon filters on it and only change my oil about every 10 to 12,000 miles it has over 400,000 on the engine and still only uses less than 2 qts in 10,000 miles
My little Kenworth that I bought new in 89 had close to a million on it when I sold it and the oil in it only got changed every 50,000 but I had a high speed centrifugal auxiliary filter on it