Will a lighter-weight oil improve fuel economy? Tom and Ray answer one 12-year-old's question.

Dear Car Talk

Dear Car Talk | Nov 01, 2007

Dear Tom and Ray:

I am 12 years old and love your column and radio show so much! We have a 2006 Honda Fit. I heard one time on your show that if you use a lighter oil, you will get better mileage. Is that true? Will using a lighter oil increase our mileage? - DJ

RAY: Yes, and we recommend switching to Newman's Own Light Italian.

TOM: In theory, if you use a lighter oil, you will get better mileage, DJ. That's because oil has a certain viscosity, or thickness. And the moving parts of the engine have to push the oil out of the way as they move.

RAY: The lower the viscosity (the thinner the oil), the easier it is for the parts to do the pushing.

TOM: Imagine that you have two saucepans. One is full of pancake syrup, and the other one is full of water. You take a fork, you put it in the saucepan and you try to move the fork from one side to the other.

RAY: Obviously, the fork is going to be harder to move in the saucepan full of syrup. Because it takes more energy to move the fork through the syrup, if you were an engine, you'd use more gasoline doing it.

TOM: But the viscosity of the oil is related to the protection it provides to the moving parts of the engine. That's the oil's primary job. Honda, which is warranting this engine for five years or 60,000 miles, has decided that the thinnest oil that's safe to use is 5W20.

RAY: If Honda felt it could use a thinner oil, and be able to advertise higher mileage, it surely would. But it has determined that a thinner oil won't provide enough protection to the engine parts. And it knows that it'll end up paying for those failed parts, in money and in reputation.

TOM: So we suggest that you use only the grade of oil recommended by the manufacturer, and no thinner.

RAY: Of course, if you want to switch to a thinner syrup for your pancakes, DJ, that's fine with us.

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