We should use an oil that protects us between the temperature range we live in. Thinner is better, both for your engine and your pocketbook. I know that putting a thinner oil in your car goes against everything you have been taught. Most of us cringe at the thought of putting a 5W-30 in our engine. First, you must understand what motor oil is supposed to do.
Oil is used to provide lubrication between two moving parts to reduce wear.
Most internal engine tolerances are measured in thousands of an inch and many tolerances are no thicker than a human hair. Oil is used to gather or absorb heat from the internal engine parts and carry it away. Oil is also used to flush the metal particles from the bearings of your engine. That initial cold start wear can equal hundreds of miles of warm engine wear. There are situations that call for a thicker oil.