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Motor Oil

September 8, 2010

A listener asks what the numbers on a bottle of motor oil mean.


A slick question about motor oil…I’m Bob Hirshon and this is Science Update.

Listener Donna Berlin of Columbus, Ohio wrote to ask what the numbers on a bottle of motor oil mean. For the answer, we went right to the source of those numbers, the Society of Automotive Engineers. Gary Pollak, SAE’s program manager of technical projects, says the numbers refer to the oil’s viscosity—the higher the number, the thicker the oil. Multi-viscosity oils have two numbers, like 10W-40. The first number, with the W, refers to the oil’s thickness in winter temperatures.

GARY POLLAK (Society of Automotive Engineers):
Naturally, at lower temperatures you’d want a much thinner and lower viscosity fluid to ensure that it will flow to all the parts of the engine.

The second number shows how well the oil flows in a hot engine, when you want the oil to be thick. And if you’ve got a science question, give us a call at 1–800-WHY-ISIT. I’m Bob Hirshon, for AAAS, the science society.