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)
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