When Checking Oil Levels, Temperature is Key

Dear Car Talk:

My daughter has a red 2013 Subaru Outback with 85,000 miles on it. She was driving from Omaha to Wichita when the "check oil level" light came on in the remote prairie of Kansas.

She dutifully pulled over at the next exit and checked the dipstick. She did it properly, I believe -- wiping it off before reinserting it and then removing it.

The dipstick showed the oil as being only half a quart low. I advised her that since it was only half a quart low, it was OK to drive the remaining 150 miles to Wichita and have it checked there.

When she got to Wichita, her friend's father checked the oil and got the same reading: half a quart low. We decided it was OK for her to drive back to Omaha and then figure out why the oil level light was misbehaving.

Her Omaha mechanic told her that there was nothing wrong with the light, and that she was indeed low on oil. Is it possible to get a "false positive" on a dipstick? If so, how can this be prevented? Thanks. -- Alan

It's absolutely possible. And, in fact, it's likely, based on your description.

When the engine is hot -- as it certainly would have been when she first pulled off the highway -- the oil is not only thinned out, it's also splattered all over the place. Including all over the inside of the dipstick tube.

So it's entirely possible that, even after wiping off the dipstick, oil from the sides of the tube got on the stick again when she dipped for the second time to check the level. And if her friend's father checked it soon after she arrived in Wichita after 150 more miles of driving, the same thing could have happened.

So most likely, she was a quart or more low when the light came on, and the mechanic was the only one who got the level measurement right.

You don't say how low the oil actually was when the Omaha mechanic checked it. If it was just a quart or so, it's unlikely she did any damage to the engine.

But if this happens again, even if the dipstick only shows half a quart low, the safest course of action is to stop at the nearest 24-hour Walmart off the highway, buy a quart of oil and dump half of it in.

Then, the next morning, check the oil level properly. When the engine is stone cold, all of the oil will have run down out of the dipstick tube, and you'll get a perfectly clean reading. As a bonus, you don't even have to wipe off the dipstick. Or burn your fingertips trying.

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