SAE0W-40 is a viscosity rating, a measure of the flow qualities of the oil. It is a rating system based on a standard, that standard being a petroleum oil without viscosity modifiers.

An SAE0W-40 matches the viscosity or "thickness" of a 40weight oil when at the engine's at running temperature, and is thicker than that when cold. The reason it is a 0W and not a 5W or 10W etc. is because it doesn't thicken as much as a 5W or 10W when cold, but it is still thicker than it is at running temperature.

As far as shear, that is a different measurement and has to meet minimum ASTM standards, has little to do with the 0W rating.

It is for all intents and purposes an SAE40 oil when running.

If it were a non-synthetic, I'd agree with you as the non-synthetic multi-viscosity oils start with a basestock that matches the xxW rating and add viscosity modifiers to help keep the oil from thinning as much when warm to achieve the hot rating (the latter number in the xxW-xx rating). Synthetics usually use a thicker basestock because it naturally doesn't thicken as much when it gets colder (Mobil 1 5W-30 was originally formulated without any viscosity modifiers, just was a more stabil viscosity and earned a multi-viscosity rating).

I don't think that Mobil makes a non-synthetic 0W-40 under any name, and all of the Mobil 1 oils are synthetics of some classification.

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