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According to the manual, we should change the lubricant when the engine is still hot to easy the lubricant flow out completely.

But from other perspective, loosening the bolt at the high temperature can make the bolt thread get damaged. It is also applied to the case when replacing the spark plug, the engine should be cold to avoid damaging the housing.

What do you think?

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Are you referring to changing oil? I see sparkplugs as a tag. –  Evan Parsons Dec 23 '13 at 18:53
    
@EvanParsons: Because It has a reference to the case of changing sparkplug as well. –  stalking is prohibited Dec 23 '13 at 18:59

3 Answers 3

up vote 6 down vote accepted

The oil pan never gets hot enough in normal operation to soften the metal of the oil pan or the drain plug, and any thermal expansion at that temperature also shouldn't be much of a concern.

For me the ideal temperature is maybe about 20-30 minutes after a drive, or when I can safely put my hand on the drain plug for a few seconds. The oil is still warm enough to flow easily, but I'm not going to get hurt changing the oil. Waiting for it to cool enough to handle means I can take the time when replacing the drain plug to ensure that the drain plug is threading cleanly into the oil pan, so I'm less likely to cross-thread the drain plug.

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I agree. One other thing I would add when dealing with warm/hot oil is to wear nitrile gloves when you do change oil. This will protect you incase your oil is unexpectedly warm. It won't catch you off guard. I know this isn't high on every bodies mind, but oil also has known carcinogens. Long term exposure can cause cancer ... At least that's what the doctors say ;-) –  Paulster2 Dec 23 '13 at 23:42

Modern cars use such thin oil (especially those that take synthetics) that I don't even worry about whether it's warm or not anymore. If it's warm it'll do a little better (and quicker) job of draining, but I don't think it's worth starting the car up for.

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Changing the engine oil while it is hot is the usual way of changing engine oil. It allows the oil to flow from the engine more easily taking less time.

Other then time-taken it will make no difference to an oil change.

Both the sump and sump plug will have the same degree of expansion, minimal, and will have no real tendancy to cause damage.

Damaged sump plugs are usually down to over tightening when refitting them, or ill-fitting tools when removing them.

Changing spark plugs on an engine with an alloy head is best done cold because of a difference in expansion of the alloy head and the steel body of the spark plug.

There have been cases of spark plugs snapping off at their hexagon on the plug body. This is usually down to the length of time new spark plugs now spend fitted in the engine, and over tightening when fitting. (How many mechs have you seen walking around with a spark plug torque wrench in their back pocket.)

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