When are you supposed to check your oil, when the engine is warm or when it's cold?

Dear Car Talk

Dear Car Talk | Mar 01, 1997

Dear Tom and Ray:

When are you supposed to check your oil, when the engine is warm or when the engine is cold?--Paul

TOM: That's an excellent question, Paul. We've always told people to check the oil level first thing in the morning when the engine is cold.

RAY: There were several reasons for this. First of all, when the engine is cold, all of the oil has had a chance to drip down from the top of the engine and settle in the oil pan, where it's measured by the dipstick. That way you're getting a true and accurate reading of exactly how much oil resides in the engine.

TOM: And second, when you check the oil first thing in the morning, you have the added convenience of being able to use your flannel PJs to wipe off the dipstick.

RAY: But a few years ago, Ford Motor Company started recommending that people check their oil on Fords, Lincolns and Mercuries when the engine was warm.

TOM: "Warm!" we said. "How can this be?" So we called Ford and they told us that they determined that very few idiots like us were going out first thing in the morning in their bare tootsies and checking the oil. Most people, they said, tended to check their oil when they stopped for gas, when the engine was warm. So they simply recalibrated their dipsticks to read correctly in a warm engine, when the oil has heated up and expanded.

RAY: "But what about all the oil that's still at the top of the engine?" we asked. "The stuff that hasn't dripped down to the oil pan yet? Won't that result in an inaccurate and greatly varied reading, depending on how long you wait after turning off the engine? How can this possibly work?" we wondered.

TOM: So we called our secret industry oil source, who we'll refer to here only as Deep Dipstick. Deep Dipstick is a widely respected auto-industry authority, which explains why he begged us not to use his name in OUR column. Suffice it to say he is intimately familiar with all issues relating to lubrication. And what did he say?

RAY: He said the amount of oil at the top of the engine wouldn't be enough to make any significant difference. "Unless the oil passages are all plugged up, you're probably talking about an eighth of a quart or less," he said. Not enough to induce you to add a quart when you don't really need one.

TOM: So there you have it, folks. The answer is; it hardly matters. So our advice is to follow the instructions in your owner's manual when you're in the mood for a really accurate reading. If it says to check the oil cold, the dipstick has been calibrated for cold, unexpanded oil. If it says to check it warm, we and Deep Dipstick hereby give you our heartfelt blessings.

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