Originally Posted by brad8266
It isnt going to make a difference.
we have a winner. there is so much science to motor oils these days with the synthetics on the market. the SAE test is performed at 40C and 100C. that gives you the cold (usually lower number, as in the '5' in 5w20) viscosity first and 'operating temperature' number second.
Technically, viscosity is defined as resistance to flow. Commonly though, we think of it as an oil's thickness. To be more specific, it is the thickness of an oil at a given temperature. The difference between multi-grades and straight-weight oils is simply the addition of a viscosity improving (VI) additive. The most common grade of automotive oil in use today is the 5W/30, which is a mineral oil refined to the SAE 5 weight viscosity range containing the usual cleaning and anti-wear additives, then blended with a VI additive that should leave it reading in the SAE 30 weight range when at the higher (210F) temperature. The advantage to the multi-weight is, when starting the engine, the multi-viscosity oil has the thickness of an SAE 5 weight, which allows the engine to spin over more easily.
follow the manufacturer's recommendation for which viscosity oil to use. they have struck the balance between the different metals that're used inside the motor and efficiency vs. wear. in other words, while you probably couldn't hurt your motor by switching viscosity, you won't be increasing performance, either. if you absolutely must spend more money on oil than the average joe, invest in royal purple. it is said to increase performance by 2-5%, which, depending on the motor, can be a significant amount.