What happens if I change the oil when the engine is cold?

Think of a cup with sediment (dirt, sand). If you shake it up real good, then dump it out, you will get most everything out. If you let it sit overnight, the dirt and sand settle to the bottom. When you go to dump it out, you dump mostly water, and most of the sediment is still at the bottom of the cup. Your oil is similar with the gunk you are trying to clean out. Allowing it to at least partially heat up lets oil circulate through all the passages and pickup any gunk you want to remove.

When I change oil, I use to run the engine warm enough, but never "untouchable hot". Never to operating temperature, that's what? 180F degrees? I let it get some temperature I can deal with. Then let it drain down all the time it takes. I usually uncap the breather and remove the stick gauge to speed up the process. The oil comes out warm but not enough to burn my hands, say, like hot shower water :)

No problem doing it cold, but better to do it hot so the oil flows easier. Proper tools will help, and so will experience. I've been burned a lot too, but that's because I'm not very adept, and have worked in "quick lube" environments where the customer is in the waiting room after coming off the highway, and I did not have the luxury of any cool-down time.

A shadetree trick that costs nothing, is to run a drywall screw into the bottom of the oil filter. After the pan is completely drained, back out the screw and drain the "reserve" quantity of oil in the filter. This mitigates most of the chance of getting burned during filter removal, and makes the job a whole lot cleaner.

Although not a direct answer to the OP's question, a piece of advice picked up from my many trade customers:

In the new Ford Ranger with 3.2 L Duratorq TDCi ("PUMA" P5AT) diesel engine, draining all the oil out can cause engine failure because the oil pump is not self priming and the oil must be drained and topped in quick succession. No sloping off for a coffee while it drains, you need that bung back in when the flow slows off, so that the oil pump still maintains it's prime. For this reason, doing it cold is better as it keeps the oil viscous enough to hang in the intake for a while.

Quality engineering, boys. Ford have replaced engines themselves because their own technicians took too long / drained too much out!!

If you want to get the most oil out of engine change it cold. The engine was hot when you last ran it. The oil ran into the pan until the engine was cold. If you change it hot the oil has not had enough to completely drain. If you warm it up you are simply putting oil back in the top of the engine along with any impurities that had already drained and settled. If your concerned stick a light cloth in the drain hole and swap. Its cold so not a problem to do this.

I am one that supports the opinion of draining the oil when cold. The oil has had hours to completely drain off of the engine parts and collect in the pan. I understand the viscosity benefits of draining hot oil more quickly, however, the idea of stirring up all of those impurities by pumping it back through the motor just to warm it up doesn't make sense. I change my oil when I have time to let it sit for awhile so time is not an issue. This is just thought-candy... Anyone willing to challenge the prevailing practice with a little research?

I’ve been contemplating this question for a while (about 40 years) and I know your suggestion of doing it hot/warm is the traditional approach, but as a Mech E and applying some fluids and materials science to it and having done some experiments, I think it’s not correct. Maybe modern mult-viscosity oils are the difference.
When multi-vis oils are heated and moved, they become more viscous - more sticky - making it harder and slower for the oil to drain from the sump and from the higher parts of the engine. Plus, after the engine runs, the oil is spread throughout the engine instead of sitting in the sump and it has to travel further to get to the drain.
I experimented on my 2013 Fusion and found it nearly an hour faster to drain the oil to a point where the oil would slow it’s draining to drip one drop in at least a minute if the oil was cold. IOW, the oil drains faster when cold. This speaks only to the time to get the oil out and assumes that the portion removed from the engine is the same at that point and doesn’t address the question of residue coming with the oil. I will experiment with my 2018 Jeep that takes 0-weight oil over the next year to see if that’s different. For what it’s worth...

Logically, the impurities and any particles heavier than the oil will settle and reside on any horizontal surfaces of the engine that the oil passes through. The majority should settle in the bottom of the oil pan where the oil collects because of gravity. As the oil sits and cools, the majority of sediment will settle in the oil pan along with the oil. This makes it more efficient to quickly drain the majority of the oil even though it is less viscous. However, because the bottom of the oil pan is not shaped like a funnel, much of the oil will simply pass over the heavy sediment as it is drained, though the initial quick flow of the large volume of settled oil may carry a fair amount with it. Think of it like panning for gold and draining the water off the slurry as the gold settles. Therefore, it only seams logical that draining hot oil will carry out more of the sediment that is suspended within it before it has a chance to settle out of the oil. If anyone has a counterpoint to this reasoning please let me know. Thanks to everyone else's viewpoints. I have read them all and appreciate everyone's contributions. PS My honours degree was in Chemistry and Energy & Fuels Science.

My entire life the only vehicles I have never warmed up before an oil change have been my motorcycles. Currently I drive a 1996 Buick Regal with a 3.8 V6. When I bought it it had 43,000 miles on in 2010. It now has 183,000 miles and is still going strong. I have a 2000 Dodge Dakota pick up with a 4.7 L V8 that has 110000 miles on it that I bought new in 2000. Also a 2012 Ford Focus with over 90,000 miles on it. These are still going strong also. That is my experience thus far on warm vs. Cold oil changes. I am in my fifties and I've been changing my own oil for many years. So that is my two cents worth. I do not agree or disagree with what anyone else has said this is just what I have done for years and it has worked out well for me.

What happens when you drain the oil when cold? Nothing...

What happens when you drain the oil when warm? Nothing...

What happens when you drain the oil when hot? Nothing...

Using the correct oil and filter along with changing oil at proper intervals and maintaining correct oil levels are important things to concentrate on. Oil temperature during change is a non issue.

changed oil with engine cold; now I believe it is the ONLY way to go. Particularly with the new synthetic oil that are designed to flow at 0 F

Do not warm up your engine to change oil if you are using multi-grade oil. Example of multi grade oil’s are 5W30, 10W40, etc. “W” means “Winter”temperature (cold). The number before “W” is the viscosity of the oil when cold. If you are using 5W40 for example, it means that the viscosity is 5 when the oil is cold & the viscosity at operating temperature is 40. Some additives that’s added to the oil expand (thickens) when heated. Viscosity number 5 is less thick than viscosity number 40. Just like the Viscosity of water is lower than the viscosity of honey or ketchup.

I agree with draining it cold. A multi viscosity oil, 5W-20 for example, will drain faster at a thinner viscosity (5w) than at 20w and all the oil in the engine will be present in and around the drain plug in the oil pan as opposed to all over the internal components of the engine after it has been running.

it seems to make sense that draining the oil cold after all the heavier particles settled in the bottom is the way to go. Modern syntethic multi viscosity oils have no problem with slow flow rate when draining out without warming up the engine. This is also a safer approach.

I would agree with cold oil change. I believe that the initial reason for a warm oil change was to coat engine parts to prevent dry start in older vehicles. With today’s milt- viscous oils and modern design and technology this is no longer a problem. Let the filter do the job and practice patience.