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Old 04-02-2016, 09:04 PM   #1
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Engine oil changes: hot or cold?

I have always changed engine oil when its hot, or at least still quite warm after running the engine. I seem to recall as a kid it was always the way it was done. I am wondering whether this was a result of higher viscosity single grade oils used back in the day.

Right now it will be more convenient to do an oil change with a cold engine, via the oil change pumpout system. Its easy to turn the pump off, come back after 30 minutes and turn it back on to get the last little bit out.

Just curious as to what most folks do, hot or cold changes. And if anyone like Ski has sound reasons for not doing cold oil changes. I've heard of particulates not draining out with cold oil but figure that if there were any particulates around then they would end up in the oil filter, so not too concerned about that.
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Old 04-02-2016, 09:39 PM   #2
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I like to change it warm after the engine has run for a bit but the oil has had a chance to drain back into the pan. Easier on the pump and the particles should still be in suspension. And those particles that may have settled out should move out with the oil easier if the oil is warm.

At least that is my reasoning.
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Old 04-02-2016, 09:40 PM   #3
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One of the biggest reasons for doing it with a warmed up engine is that cold oil is thick, and it can burn up your oil change pump. Been there, done that. Otherwise, not a big difference cold or warm.
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Old 04-02-2016, 09:59 PM   #4
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Originally Posted by Ski in NC View Post
One of the biggest reasons for doing it with a warmed up engine is that cold oil is thick, and it can burn up your oil change pump. Been there, done that. Otherwise, not a big difference cold or warm.
I agree with Ski, but its not because I know any better. I visualize the process. Water will settle to the bottom. So a cold change should get it out. Particles will settle so they should also be removed. All residual oil should drain down on a cold pump out. So any suspended particulates that don't drain down hot should hit the bottom cold.
Having said all that I pull hot samples for analysis prior to oil changes so it is all mixed up.
But what the heck, I am guessing.
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Old 04-02-2016, 10:10 PM   #5
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My line of reasoning is the particles that may settle to the bottom, as well as the oil, are more likely to move, flow out easier if the oil is warm as opposed to how cold thick oil flows.

But in the long run it probably makes little difference.
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Old 04-02-2016, 10:37 PM   #6
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I always do it hot, but see what the manufacturer recomends. I've never seen one that said cold.
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Old 04-02-2016, 10:59 PM   #7
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I have not found a manufacturer recommendation for warm (or cold). But I'll likely do this change warm after all because I want to do an oil analysis, and warm is going to be better for obtaining a good sample.
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Old 04-03-2016, 08:15 AM   #8
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15% or more of modern oil is the additive package.

A good portion of that is a detergent.Esp with newer oils.

Detergent keeps the big crud in suspension so the oil filter can grab it.

The fine stuff that passes the filter still requires removal for longest engine life.

A long 5-8+ hour run allows the detergent to re grab the fine stuff stuck to the sides and bottom of the oil pan and get it back in circulation.

This happens with every long engine run , weather you are changing the oil or not.

"Best Practice" would seem to remove it every time the oil is changed.

Or use a centrifugal bypass filter system and catch it as its formed.

Might be worthwhile to take the extra effort after a long days run to change the oil hot and dirty ,
so the beast can "live long and prosper".
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Old 04-03-2016, 08:33 AM   #9
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Like Ski and others, I start changes after a short run to heat up the oil. We use the installed pump-out system.


FWIW, I usually only do the "outie" when oil is warm. I usually don't do the "innie" (fill) and filter changes until the next day, when the engine room and filters are a more comfortable temperature.


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Old 04-03-2016, 09:26 AM   #10
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I don't believe it makes much difference as long as the pump can handle it easily.
I used to do gassers hot as the cheap pump I had wouldn't handle cold very well.
Current X-Change pump handles cold / warm very easily and is away from engine so heat not much of a problem.

It might make more of a difference if you are pulling a sample for analysis...

Blackstone's sampling recommendations include....
"Run the engine (don't just idle) before you take the sample to get the oil at operating temperature — about 20 minutes should do it.
Getting the oil up to operating temperature should help cook out any normal moisture or fuel build-up."


I'd be interested if anyone has done it both ways and found any difference in the amount you get out? (w/ similar dip stick oil levels to start)
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Old 04-03-2016, 10:24 AM   #11
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I change when hot. That is .. run for several hours not just run a bit at the marina. The additives, carbon and "dirt" (foreign matter) is much more in suspension when the engine is hot. And draining quickly before much "stuff" settles where it will remain in the engine is a goal for me.

My Mitsubishi Owners Manual says "Immediately after shutting down the engine (when the oil is hot) remove the oil pan drain plug." Changing when cold or warm is not mentioned.

I think many or most likely most manuals will say do it hot. Cars, motorcycles ect. Probably hard to change your lower unit OB oil hot though. But if you really need or want to change oil cold I don't know to what degree you've really done what needs to be done. The biggest reason to change oil is to remove the carbon and acids that form from moisture over time. But carbon (the black stuff) is an abrasive that wears engine parts like bearings and that's job #1 as far as I know.

So definitely change hot.
And IMO cold thick oil being a PITA has nothing to do w it.
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Old 04-03-2016, 11:14 AM   #12
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Originally Posted by ranger42c View Post
Like Ski and others, I start changes after a short run to heat up the oil. We use the installed pump-out system.


FWIW, I usually only do the "outie" when oil is warm. I usually don't do the "innie" (fill) and filter changes until the next day, when the engine room and filters are a more comfortable temperature.


-Chris

Thanks. That's one of those clearly obvious things once I see it, but never thought of it before.

And my oil change pump will not pump cold oil either.
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Old 04-03-2016, 11:49 AM   #13
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Oil analysis lab don't seem to care...I don't care except hot oil pumps and pours easier.

As for engine longevity...like most discussions here....flyshi*.
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Old 04-03-2016, 12:17 PM   #14
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I've done it both, my old clam crusher is still happy. Meh.
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Old 04-03-2016, 12:25 PM   #15
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I have always done it on a warm engine just because it is a lot easier to pump the used oil out of the dipstick tube when it is warm. I never have given it a lot of thought however. The arguments for doing it on a hot (instead of just warmed up as I have done) engine make a lot of sense.
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Old 04-03-2016, 02:29 PM   #16
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I often do it cold because my pump will move cold oil with no issues. And a cool engine room is more comfortable to work in.
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Old 04-03-2016, 03:18 PM   #17
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I often do it cold because my pump will move cold oil with no issues. And a cool engine room is more comfortable to work in.
Cool is good!

But until I buy an electric pump...I work up the same amount of sweat....hot ER or pumping molasses.....
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Old 04-06-2016, 06:48 AM   #18
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Quote:
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Cool is good!

But until I buy an electric pump...I work up the same amount of sweat....hot ER or pumping molasses.....
Has anyone definitively proved whether hot oil is thinner than cold. One just assumed it was, but when you think of the purpose of the various elements in multigrade, which many of us use, (yes, I know Lehmans were originally run with 30w single grade, so don't bother repeating it), then it is even possible, if they do what they claim to, the oil might get thicker when it gets hot..? I never thought much about that.

This suggests that the above is not correct, and the assumption that cold oil, even a multigrade, is thicker when cold, is in fact correct.
Bugger, I was hoping not.

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Old 04-06-2016, 06:52 AM   #19
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Multigrade oil has a flour like substance that gets thicker as it heats.

In your galley take a cup of oil and pour it thry a small funnel and time it.

Heat the oil in the microwave and repeat.

Do the same with single weight and watch the difference.
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Old 04-06-2016, 07:18 AM   #20
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I have definitely proven it to myself.


Take any oil cold and pour it...it doesn't splatter....heat it to 180 and it sure will.


No science..just practicality.
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