the reason why they call 10w-40 or 5w30 energy conserving oil are the follow

1. when your engine is cold metal expands, so its easier for the engine to crank. it lessens the damage to ur motor.
2. the letter W means winter inwhich in motor term all weather. u can use it anywhere.
3. ones your engine reach operating temp the oil expands to what number is next to W whether its 30 or 40.
4. the numbers you you see in the motor oil is the grade of the oil and not the temp. inside the engine temp. develops upto 2500 degrees farenheit.
5. synthetic oil last longer than mineral oil but they have the basic same composition.