Why warm up an engine before an oil change?

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Tom

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Something we were always taught to do was warm up an engine before draining the oil, right? Doesn't the viscosity of multi-grade oil increase with temperature? So oil would drain from a cool engine more freely than from from a warm engine, right?

Dumb question, and one I've wanted to ask for a very long time -  why do we warm up our engines before draining the oil?
 

Ron

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Nope hot oil will flow more readily than cold oil.  Modern synthetics like Amsoil the viscosity changes less the the old Dino oil but there is still some viscosity change.
 

Tom

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Ron,

I should have clarified I was talking about multi-grade oil. Synthetics I know nothing about.
 

Just Lou

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Tom, oil does not thicken with heat.  10w30 only means that it should never act thinner than 10 or heaver than 30. 
 

Tom

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OK thanks Lou. If oil doesn't thinken with heat, does it get thinner? If not, why do we warm up engines before draining oil?

TIA
 

Ron

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All oils become thinner to some extent as they are warmed up.  You probably have 10W40 oil in your diesel  when the oil is very hot the visccosity is less or thinner than when cold. 
 

caltex

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A friend of mine tried to change to oil in his boat with a 12v oil pump, without warming up the oil.  The pump wasn't strong enough to pull the oil up through the pump hose in the dipstick channel. No big deal you say, but when he tried to pull the hose out it wouldn't come. Some disassembly was required on the CAT 3208.
 

Howard R

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So you can burn your fingers with the hot oil when you take the plug out of course!  ::)

Seriously Tom, be it right or wrong, I've always understood the reason is to allow a quicker and more complete draining of the old oil and any contaminants in the pan.  Not totally unlike waiting for the black tank to be mostly full before dumping ... volume and flow.

Think about a bottle of syrup ... just about empty ... need those last few drops for your morning pancakes.  If you just took it out of the refrigerator you'll be eating cold pancakes by the time the last few drops of syrup come out.  But put that bottle in the microwave or warm water for a bit and the syrup just pours out and the bottle is almost instantly empty and relatively clean.

Oh, and watch the syrup bottle ... that can burn the fingers also!  ;D

Howrad
 

Tom

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Howard Rawley said:
I've always understood the reason is to allow a quicker and more complete draining of the old oil and any contaminants in the pan.

Thanks Howard. That's what we were always told and is the reason I'll keep warming an engine before draining.

Unfortunately, the the syrup analogy is lost on me as I don't use syrup on anything. But I'll check next time Chris uses it on her pancakes  ;D
 

King

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The way it was described to me...
Oil thickens as it gets colder and thins as it gets hotter.  A 30 weight at zero will hardly pour, and at 300 degrees is like kerosene.  A 10W-30 should behave like 10W at zero degrees and like 30 weight at 300 degrees.  It still thickens as it gets colder and thins as it gets hotter, but not as much.
 

Tom

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King said:
It still thickens as it gets colder and thins as it gets hotter, but not as much.

That's it in a nutshell King.

For some reason, I always thought that 10W-30 meant something different, although intuitively I believed that a single grade oil thickened with increasing temperature.

The following extract from Valvoline's web site clarifies what previously appeared (to me) to be a conflict:

"Multigrade oils typically begin as base oils, such as 10W. Then viscosity-index modifiers (polymers) are added in an effort to stabilize the viscosity. This allows a 10W40 oil to flow like a 10W at cold temperatures and a 40W at higher temperatures. In other words, multigrade oils are formulated to pass viscosity tests across a range of weights. For example, 10W30 meets the requirements for 10-weight at cold temperatures and 30-weight at high temps. The multigrade oils' viscosity modifiers are long-chain molecules that lessen the change of viscosity with temperature variance."

For the engineering types, here's a Wiki on the temperature dependence of liquid viscosity.
 

jambon

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I always drain the oil in my Cummins M11 when it is cold.  I leave the drain plug out overnight so all the oil drains.  Cold thicker oil has a better chance of carrying all the impurities out with it.  It also doesn't burn my hands when I get oil on them.  After the engine has set for days all the oil thats going to drain down in the pan is already there.  Another good reason to drain cold.  JMO
 

John From Detroit

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Re: 10W-30 getting "thicker"

How thick an oil is depends on two things, 1: It's viscosity and 2: Temperature (I'm talking clean oil here, impurities are a #3 but for this discussion can be overlooked)

As oil heats up, a single viscosity oil, in fact ALL oil, thins out.  If it thins out too much then you no longer have sufficient lubrication.

By the same token as it cools down it gets thicker, Thicken it too much and 1: Your vehicle becomes very, very, very hard to start and 2: you do not get needed lubrication

So they put additives in it which tend to affect the way it thins and thickens.

Thus a 10W-30 oil, at low, Winter temperatures, acts like 10 weight (thick, but not too thick)

And at normal operation temperatures of say, around 200 or more degrees, it acts like a 30 weight (Which at that temperature will be thin, but not too thin)

Very confusing if you don't work with that kind of thing.... 

What can I say... The kind of things you learn reading /Farm Journal/ and /Mechanics Illustrated/ and /Popular Mechanics/ and the like
 

Just Lou

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Why is it that    "one line answers"  i.e.

(Oil thins with heat),
(10W40 adequately describes the operating parameters of a multi-grade oil)
etc...

never suffice on this forum?
 
Must we always continue the discussion (debate) until John from Detroit finds it necessary to post his usual enlightening pontification, obfuscation and bloviation, in no less that one hundred words before we say,

Oh Yea! Now I See?
 

Tom

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Things would be rather boring around here if all the discussions ended after a single 1-line answer.
 

Just Lou

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I whole heartily agree Tom.
 
I did forget to add a big... LOL to my last post.

I'll take this opportunity to say that You, Your staff and ALL the regular contributors, do a great job with this forum. 
I've made it required daily reading at my house.

Still, don't ever ask some folks here what time it is unless you really want to know how the watch was put together...
but that ain't all bad either..............LOL

Keep on keepin' on.  lou

 

Tom

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OnaQuest said:
I did forget to add a big... LOL to my last post.

Hi Lou, you didn't add it, but I still read the big  ;D in your message.

don't ever ask some folks here what time it is unless you really want to know how the watch was put together

Now isn't that a fact!  ;D

Glad to hear the forum is part of your daily reading and thanks on behalf of all our volunteers for the kind words.
 
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