Today's motors are bigger, lighter, and more create more horsepower per cubic inch than previously and hold less petroleum volume, stressing the motors oil to the maximum.
Good motor oil must be able to maintain a constant viscosity when exposed to temperature changes. You can find the best motor engine oil through the internet.
It is also very important that the oil be able to maintain its proper viscosity for the entire duration of its intended drain interval.
A oils pour point tells you how well it handles cold temperatures without gelling (solidifying).
Oils that contain too much paraffin; a wax found in petroleum motor oils tend to gel at low temperatures. Oils with poorly designed additives or low-quality viscosity index improvers will also have this problem.
Modern engines have tight tolerances and need engine oil to flow quickly at low temperatures to minimize wear at startup.
At high temperatures and pressures, motor oil must be able to resist shearing forces. The condition known as shear is where the oil is literally torn apart on the molecular level causing it to drop out of its proper viscosity range.
This brings us to the subject of volatility and heat-related deterioration. At high temperatures, an oils lighter components can volatilize and boil off. This is especially true of petroleum-based motor oils which have a mix of hydrocarbon molecules of various sizes.