Clearing up common misconceptions with Oil weights

[Decreppo]

There are two misconceptions out there I can't quite get a straight answer on from searching this and other forums. They are connected so I will keep them to one post.

1)
All over this forum and others I have seen people saying they run a heavier grade oil for the summer months, and a lighter weight oil for winter months. NOT the winter grade number, but the second, oil weight number.

for example, I see people saying that they run Xw-40 in the summer and Xw-20 in the winter. The same can be said for geography. People say they run Xw-40 because they live in Arizona, or that they run Xw-20 because they live in Canada.

Notice how we are ignoring the cold-start/winter weight (0w/5w/10w) for now.

Correct me if I'm wrong but don't all oils run at the same temperature when a car's engine is fully warmed up therefore, no matter where you live, no matter what time of year it is, the second number should stay the same for your oil year round and indiscriminate of location?

2)
the second question I have is in regards to using 10w30 vs 5w30 oils. Lets put aside the debate on whether or not one is better than the other for shear strength or NOACK or whatever etc. etc.

MY question is if you live in a temperate climate, I live in Virginia, is it better for your car to use the 5w in the winter for better cold starts? It got down to 12F here; this is within the range of what a 10w is supposed to be good for. So is the 5w or even a 0w going to reduce engine wear before the engine gets up to temp? What benefit do I gain if the cold start engine temperature is above what 10W is rated for? Is a 0w-40 oil better at 32F than a 10w40 oil?

Would love to hear these cleared up.

 

[Jetronic]

1) No, while coolant temps are fairly tightly regulated, the same isn't true for oil temps. Loads of engines just depend on air passing by the sump for cooling. Though that is changing with modern turbo cars, they are much more likely to have another form of oil cooler aswell. With an easy 100°F swing in ambient air temps in a lot of places between summer and winter, I can definitely see oil temps changing 40°F or more between summer and winter. And an xW-20 running 40°F cooler than an xW-40 is pretty much the same operational viscosity. It all depends how long your typical run is, if there's a benefit or not.

I don't drop viscosity for winter, as i'm wanting the thicker oil to get the engine up to temp sooner, to evaporate water ASAP.

2) It's more complicated than you want to frame it. But if all else is the same, there's no benefit for a 0W-xx at those temps. But not all else is the same, usually.