Here in North America, the auto makers and the API/SAE have perpetuated this myth that "one oil is good for all seasons and all driving conditions."
Most of that rationale has to do with the EPA's CAFE (Corporate Average Fuel Economy). CAFE testing is such that a fraction of a MPG is applied fleet-wide, and the formulae allow the car maker to build CAFE "credits" that offset poor MPG results in other cars/trucks.
If you examine the viscosity/temperature charts for other makers in other parts of the world, viscosity recommendations vary quite widely based on temperature.
In most cases, you are not supposed to use a 5W-30 in temps exceeding +10 C / +50 F. For consistent summer operating temps of +20 C / +68 F, most makers recommend the following viscosity:
10W-40, 15W-40, 15W-50, and 20W-50.
Remember that the so-called "energy conserving" oils will effect fuel consumption by 1-3% max. Usually on the lower end, say 0.5 - 1.5% max.
What does this mean for the average driver?
Say you have hypothetical fuel economy like follows, then increase it by 1.5%:
10 MPG + 1.5% = 10.15 MPG
20 MPG + 1.5% = 20.3 MPG
30 MPG + 1.5% = 30.45 MPG
40 MPG + 1.5% = 40.6 MPG
50 MPG + 1.5% = 50.75 MPG
80 MPG + 1.5% = 81.2 MPG
As you can see, in most cases the fuel economy "improvement" falls within expected statistical variability and will be unnoticed by the average driver.
Though this is supposed to make you feel good about driving something like a Ford Excursion with a V10 motor.
If you truly want to improve fuel economy, try the following:
1. Buy the smallest car practical, maybe a 3 cylinder with 1.3 litre max displacement
2. For other vehicles, especially pickup trucks, run the smoothest highway all-season tire possible. No lift kit, no aggressive off-road tires.
3. Keep the tires properly inflated the the wheel alignment in spec.
4. Is the air filter clean? Or is it plugged?
5. Spark plugs clean?
In my own daily driver, a 2000 GMC Sierra that sees towing duty in summer of +8,000 lbs, I run the following oils:
Winter: Mobil 1 0W-30
Summer: Mobil 1 10W-30, soon switching to Mobil 1 5W-40.
In my climate, I can see winter lows of -42. In summer, +35 C is nothing unusual. If I try to run the 0W-30 in temps hotter than +10 C / 50 F, my truck's Vortec motor makes a lot of valvetrain noise.
Most of that noise goes away running 10W-30.
A final thought regarding an oil viscosity: this is rated at 100 C and "low" shearing force, and is a Kinematic viscosity.
In most parts of the motor, you will see MUCH higher temps and shearing forces. The HTHS (High Temp High Shear) is quite a bit different than Kinematic viscosity and reflects the effect of shear/temp on oil viscosity.
For example, the oil film in the cylinder crosshatch next to the oil control ring may see temps of +600 F.
Jerry
