[Msquared]

Originally Posted by Boost Creep

It's still a 0w oil with an additive package to make it act like a 40 weight when hot. I'd rather start with a more substantial base.

In the case of Mobil 1, I don't believe that's really true. If talking about a natural "dino" oil, it is true. I believe it is also essentially true for Group III "severely hydrocracked" oils that call themselves "synthetics," even though they really aren't. Most grades of Castrol used to be like this, and they may still be. But a true Group IV synthetic like M1 has an engineered molecule that requires less (or no?) viscosity modifiers to act like the thicker viscosity at high temps.

My car lives in a garage so cold flow is less critical, never really gets below 45º in the dead of winter.
The engine was designed and speced to work with 5w so no issues.

Now if it lived outside and was a daily driver, then in the winter I might think about running it, but as it stands, no thanks.

"Cold flow" means the resistance to flow at temps lower than operating temps. It's measured at 40C (104F). So if your oil is at 70F, the low viscosity number is still the number that applies. IOW, it doesn't have be below freezing for a low-cold-viscosity oil to help reduce startup wear. 0w oils protect your engine better at startup, even if it isn't cold outside. If you have two oils that protect identically well at high temps but one is thinner at lower temps, you would always get better wear performance from the one that's thinner at low temps.

Also, you have to understand that the nominal multi-viscosity rating doesn't mean that a 5w40 is only 1/8 the viscosity cold as it is hot. That's not what the rating means. It means it acts like a straight 5w oil at 104F and it acts like a straight 40w oil at 212F. It's still much thicker at 104F than it is at 212F!

That video you linked actually says these things, in a roundabout way. But you can also just compare the data sheets for Motul 5w40 8100 X-Clean (a very good Dexos 2 oil) to Mobil 1 ESP 0w40. The difference between them in Kinematic Viscosity @ 40C is pretty massive, with the much better-flowing M1 providing a lot better flow and protection at 104F than the Motul. Also note that each is roughly 6x thicker at 104F than they are at 212F in absolute terms, so your concern about the 0w40 being too thin at low temps is not well founded.

The fact is that any Dexos 2 oil has to meet the most stringent wear requirements ever required by a manufacturer. And if you look at the data sheet on the M1 0w40, it's extremely good in all pertinent categories. With its superior cold flow properties, a 0w40 that meets Dexos 2 requirements is going to be better for any street-used engine that sees lots of cold starts than will an otherwise-equivalent 5w40.