Choosing the correct oil viscosity can be an important factor in determining the life expectancy and performance of your engine. Using the correct-viscosity oil can also keep the engine running at its peak efficiency, and plays a small part in the overall fuel economy of your vehicle. Modern engines have very different viscosity requirements over vintage engines, and manufacturers invest a lot of time and money to determine which mixture of oil and oil additives will work best with their engine components. This is why you often see original equipment recommendations for new weight ranges that were unheard of a few decades ago. These new formulations will eventually lead to the replacement of non-detergent straight-weight oils as well as some of the old, reliable multi-viscosity oils of the last five decades.

Simply stated, viscosity is merely a measurement of oil and other fluids' resistance to flow. This flow can best be described as the difference between pouring water (with a viscosity of 1) through a funnel and then molasses (which has a viscosity of about 2,000) through the same funnel.

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