Understanding Oil Numbers


At AAAA, we work around a lot of oil. We always check it when we start cars up, replenish if a car is down, and change it upon customer request.

We’re around it so much that sometimes we take the stuff for granted…which is why I wanted to revisit something very common to us but kind of perplexing to most: Those weird oil numbers.


Im talking about 10w30 5w30 10w40, etc.

Many people think that the ‘W’ stands for ‘weight’ because those numbers on either side measure the oil’s weight or viscosity.  As they numbers get bigger, the oil is more viscous. More viscous oil can lubricate an engine more efficiently. However a less viscous weight will allow the motor to move with less resistance and can provide better fuel economy. This is why some hybrids (like the prius) recommend a 0w30 oil.

But back to the ‘W’. The W actually doesn’t stand for ‘weight’, it stands for ‘Winter’.

For example on a 10w30 oil, it behaves like a 10 weight oil in the winter and a 30 weight oil in the summer.

The idea here is that you want a light oil in colder months so that the engine parts can move more freely when the engine is cold (before it warms up).

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