Does thinner oil really move faster?
Freshman User
Joined: Nov 2002
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Does thinner oil really move faster?
I have been told and generally believed the idea that thinner oil moves quicker in cold weather than a thicker oil. I have been having questions about that lately and here is why. Most oil pumps I have seen pictures of (I'm not an auto mechanic) are gear pumps. A gear pump is what is called a positive displacement pump, meaning between the gears is a fixed amount of area when the pump rolls over that area is displaced, that amount of oil moves from point A to point B. Now the viscosity will determine the amount of pressure the pump generates as it moves that oil from A to B but the very same amount of oil will flow or the pump or something else will break. In very high viscosities the pump could lose suction as the fluid cavitates, resisting being pumped. I'm starting to think this is the thought process being the Cold Crank viscosity requirements and not that the oil will actually move quicker.
My thoughts, interested in what other have to say.
Kevin
My thoughts, interested in what other have to say.
Kevin
Post Fiend
Joined: Feb 2001
Posts: 6,624
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From: Missouri
Does thinner oil really move faster?
You are on the right track but not correct and here's why. Oil pumps have a spring in them that will only allow so much pressure before they start bypassing oil. If you are running 80W90 you will be pumping the same volume through the pump as if you were running 10W30 except a lot more of it will be pumped out the side and not into your engine. Thicker oil under the same pressure will move slower. Also there are many small passages in your engine such as th oil galleys in the pushrods (depending on the engine) which won't be favored much when trying to push thick oil through them. Also thicker oil takes more power to pump. At idle you will notice more pressure with thicker oil due to the speed of the pump while at idle but it won't be moving any faster at that presure. If you have ever had a problem with an older engine having lifter rattle at startup you will know that thicker oil takes longer to make them stop.
Elder User
Joined: Apr 2002
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From: Kentucky
Does thinner oil really move faster?
I learned many years ago while taking pump operation classes at fire school, pressure and volume are oppositely related. I'd rather have more volume than pressure on a cold morning by using a thinner oil.
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Joined: Nov 2002
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Does thinner oil really move faster?
Oil, like any fluid, will take the path of least resistance under pressure. Not all of the pathways for that oil are of the same diameter and length. The smaller diameter and longer ones will suffer reduced flow when the oil is too thick to be pressed through them because it is cold. That's the true value of thinner oil. It will fire through those pathways even though it is cold. Getting oil to the surfaces that require lubrication is the object of the exercise. The pump simply pressurizes the columns of lubricant.
So, perhaps the answer to your question is not that thinner oil moves faster but that it moves more readily.
Good question, this one.
So, perhaps the answer to your question is not that thinner oil moves faster but that it moves more readily.
Good question, this one.
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Does thinner oil really move faster?
Follow up question is the oil pressurized all the way to the OHC bearings, or does it just kind of fill up everything after the main bearings?
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Post Fiend
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From: Metro Detroit (Redford)
Does thinner oil really move faster?
>I learned many years ago while taking pump operation classes
>at fire school, pressure and volume are oppositely related.
>I'd rather have more volume than pressure on a cold morning
>by using a thinner oil.
Yeah, somewhere I read that high oil pressure is not necessarily a good sign, that if the oil is moving fast, the pressure will be lower and that really high pressure might mean poor oil flow. Apparently it is a balancing act between too low and too high.
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>at fire school, pressure and volume are oppositely related.
>I'd rather have more volume than pressure on a cold morning
>by using a thinner oil.
Yeah, somewhere I read that high oil pressure is not necessarily a good sign, that if the oil is moving fast, the pressure will be lower and that really high pressure might mean poor oil flow. Apparently it is a balancing act between too low and too high.
INLINE SIX POWER!
'95 F150 XL300 Cubic Inches of Low RPM Truck Torque! And twin-I-beams too!
"Drive a stick young man! There'll be time for automatics when you're old and unable."
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Senior User
Joined: Oct 2002
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From: USA
Does thinner oil really move faster?
Once the oil has been pumped, I believe gravity plays a role and a thicker oil will not flow as well to lubricate those parts that depend on gravity flow. Some parts may depend on getting oil from splashing and thicker oil doesn't splash as well either. Ok, I'm no mechanic so I could be totally off base 
This is an excellent question though.
This is an excellent question though.