N.C. Division of Pollution Prevention and Environmental Assistance
Re-refined Oil

What is Oil Viscosity?

Viscosity in motor oil is the index improvers (additives) that modify the oil so fluid flows more consistently over a wide temperature range.

Ideally, oil should be thin enough to flow easily when an engine is cold and remain thick enough to protect an engine when it's hot. Automakers specify grades according to the temperature range expected over the oil-change period. The lower the number, the thinner the oil and the more easily it flows.

In 5W-30 oil, for example, the two numbers mean it is a "multiviscosity" or "multigrade" oil that is effective over a range of temperatures. The first number, 5, is an index that refers to how the oil flows at low temperatures. The second number, 30, refers to how the oil flows at high temperatures. The W designation means the oil can be used in winter.

A popular belief is that 5W-30 oils, despite their designation, are too thin to protect vital engine parts when they get hot. However, laboratory tests measured the viscosity of oils under high-temperature, high-stress conditions and found essentially no difference between 5W-30 oils and their 10W-30 brand mates. But at low temperatures, the 5W-30 oil flowed more easily.

Viscosity grade is important, so be careful. Recommendations vary with the make, engine, and model year of the car, so check your owner's manual. And always look for the starburst symbol on the label, indicating that the oil meets API (American Petroleum Institute) requirements.

Oil does not wear out, it just gets dirty. Be sure to look for re-refined oil with the starburst API symbol in the viscosity you need.

Resource: Consumer Reports July '96

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