Garak;
I'm not sure about SAE marketing rules and how they apply to mono-grades, but if they have a W grade such as your example above, 20W20, the lowest winter grade that an engine oil passes must be the 1st number. If that is the case, many of the SAE mono-grade 20s would either have SAE 20 on the label, or some unrecognizable grade such as 15W20 and 10W20. Petro-Can dropped their Duron SAE 20 last year, the CCS passed 15W, the pumping 10W and cold flow very close to 5W. If marketed as a multi-grade, 15W20 might have been on the bottle, depending on rule interpretation.
Jetronic;
I agree that all metals found in UOAs are not wear items. I noticed a trend that winter samples showed some metals higher than summer samples and others swapping places. For example aluminum could be high in the winter and low in the summer. Iron and copper might swap places etc.
I came up with the notion that an engine when shut off hot in the winter, then cooled to a very low temperature overnight experiences acid and water condensation on engine parts, causing microscopic rust to form on the cylinder walls and maybe aluminum oxide from blocks and cylinder heads.