Which is better: changing your oil when the engine is hot or cold?

Dear Car Talk

Dear Car Talk | Jan 01, 2011

Dear Tom and Ray:

I always change my own oil on my cars. I let the car sit overnight and change the engine oil the next day. My daughter said that it would be better to change the oil when the engine is hot, like when I get home from work. So ... which is the preferred method?

-- Will

RAY: Your daughter's right, Will. The reason we change the oil when it's warm is because it flows better and comes out faster.

TOM: It's not a big deal. But when the oil is cold, it's more viscous (it's thicker), so it drains out more slowly. And even if you're willing to wait longer for it to drain out, it's likely that a bit more oil will remain coated on the inside walls of the engine when you're done. Not a lot more than if the oil was warm, but a little more.

RAY: So I'd say a warm oil change is a little bit better.

TOM: Now, there is one major disadvantage to a hot-engine oil change: It hurts! First of all, when you remove the drain plug, if you don't know what you're doing, hot oil can run down your arms. And it's not as pleasurable as the room-temperature arm grease you get from eating one of those delicious In-N-Out (of the cardiac unit) burgers.

RAY: A lot of today's cars also have oil filters that are really buried and hard to reach. So the chances of burning yourself while trying to unscrew one of those is quite high.

TOM: So, for that reason, here's what we recommend. First, don't do the oil change when the engine is red-hot. Let it sit for half an hour or an hour before starting your work. The oil will still be warm enough to flow easily, but the exhaust manifold won't be glowing red and 800 degrees.

RAY: Second, wear a protective glove, like a welder's glove or a large kitchen mitt (they make some with fingers) to protect your hands and forearms.

TOM: Or just change the oil when the engine is cold. You'll have to wait a little longer for it to drain if you want to get most of the oil out. And a small amount will be left behind. But you'll still get 98 percent of the benefit of the oil change. And you'll be able to keep all of your arm hair.

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