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Dear Car Talk: I have a Chrysler 200 that is new enough to keep me informed of how much oil life remains.

Can I trust it?

It's been 6,000 (easy) miles and 15 months since my last oil change (synthetic), and my car is saying that 25% of the oil's life is left. I'm inclined to believe it since I think that automakers are overly conservative regarding oil change intervals.

Should I change the oil when the car says 5% left? 10%? Or do you recommend a mileage or time interval? -- Jeff

I'd trust it, Jeff. If we just do an "order of magnitude" check, synthetic oil can easily last 7,500 to 10,000 miles before needing to be changed. So if you've gone 6,000 miles and have 25% left, you're on track for an oil change at 8,000 miles.

That's right on target.

In case you're interested, the oil life monitor in your car is not actually "testing" your oil. It's not taking a sample and sending it out to the lab while you sleep, dreaming about a new Honda. The oil life monitor is measuring the conditions that affect the life of your oil. It plugs them into an algorithm and constantly produces an estimate of how much longer your oil should last. From the car's computer, it collects information on things like the number of starts (individual trips), the engine temperature variations (driving conditions), and the number of miles you drive.

Over the years, engineers have created algorithms that are pretty darned accurate in predicting when your oil is spent. Remember, they have incentive to make sure you change your oil on time. If they're wrong, and you're under warranty, they could owe you an engine.

I'd say when you get down to 10%, it's time to make an appointment for an oil change. It's not an emergency at that point. Your oil is still fine. But it's like getting down to an eighth of a tank of gas; you want to know where a gas station is at that point.

Got a question about cars? Write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at www.cartalk.com.

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