Today: An idle question when it comes to oil changes.

Dear Car Talk

Dear Car Talk | Jan 01, 2007

Dear Tom and Ray:

You guys are great. Thanks for all your knowledge and insights. My question: My vehicle's manufacturer suggests changing the oil every 5,000 miles. If I were averaging a speed of 60 mph for that 5,000 miles, it would take just over 83 hours to reach that mileage. I have installed an hour meter on my vehicle so that I know to change my oil every 83 hours. Does the manufacturer figure in idle time (caused by heavy traffic and such)? I drove an 18-wheeler for 11 years, and idle time on a big rig is astronomical. That is when I came up with this crazy question. Am I actually crazy, or overprotective, or do I simply have too much free time on my hands? -- Dale

TOM: All of the above, Dale.

RAY: Actually, you're not crazy. Five thousand miles is the manufacturer's estimate of when the oil will need changing, based on the average person's driving and idling time. But it's just that: an estimate.

TOM: And you're right. The time your engine spends idling certainly does contribute to the breakdown of the oil. Which is why vehicles that idle a lot, like 18-wheelers, taxis and police cars, get their oil changed more frequently than your car does.

RAY: If you look in your owner's manual, you'll see that they change the estimate if you use your car abnormally.

TOM: Does living in your car constitute using it abnormally?

RAY: Yes, but not with regard to oil life. Your owner's manual probably lists two different oil-change intervals. One is for "normal" duty, the other is for "severe" duty. Severe duty is defined as operation in extremely hot or cold weather, towing or using the vehicle like a taxi (stopping, starting and idling a lot). And the severe-duty oil change is more frequent, to account for that extra wear and tear.

TOM: Even better than those estimates, a number of cars' computers now actually calculate when the oil needs to be changed. They do it based on an algorithm that takes into account idling time, engine temperature and driving conditions. So your idea is perfectly logical, Dale. It's just another way to estimate oil life.

RAY: But you might be overdoing it a bit. It would take you 83 hours to reach 5,000 miles IF you were driving at 60 mph. But you're not driving at 60 mph all day, every day. If you were, you'd have a hell of a time getting out of your driveway without wiping out your mailbox and your neighbor's prized Uzbeki tulips.

TOM: Assuming you do a normal mix of highway and city driving, it would probably take you more like 100 to 125 hours to reach 5,000 miles.

RAY: I'd recommend that you go with 100 hours, Dale.

TOM: Based on what algorithm?

RAY: Based on the number 100 being easier to remember than 83.

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