Hole in Manifold Finely Fixed
Otay, here iz my too centz
This is how I make my living, I know this stuff pretty good.
The "w" in 10W30 stands for winter
The multi viscosity oils have a VI, "Viscosity Index" improver blended into them, when a 5W30 oil is cold, it flows like a 5 weight, as the temp rises, the vis comes up to a 30 weight. Very important for cold start ups.
In most climates, a 5W30 is what the OEM recommends although some like Ford and Honda are going with 5W20 and even 0W30 or 20.
They are under pressure from Uncle Sam to get the CAFE, (Corporate Average Fuel Economy) up for the entire production line in order to get incentives and avoid penalties. So they are enginering the engines to tighter tolorance and less friction and are asking the oil to handle much more.
You can, and I do run a 10W30 year around in my climate. You will not hurt anything by running a 10W40 one oil change. Most cars requiring a 10W40 ceased production in the mid 80's.
If I lived in extreme northern climates I would run full synthetic, the flow characteristics are outstanding compared to mineral base lubes.
I drive on-hiway 80% and have lab sampled my oil at 3000, 3800, and 4000 miles, the additive package was still in fairly good shape at 4000 and the oil was still suspending the contaminates. I change my oil around 4000, but don't get bent out of shape if I run to 4500. Driving conditions and engine performance mean alot as to how far you can go.