Cooler engine temps with higher viscosity?!

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Hello all, I've been experimenting a little bit with my W140.
When I purchased it, it had Toyota 10W30 in the sump as the PO got it serviced at the local Toyota dealership. It was overfilled about a litre which is a big no-no on these M104s and it even says so on the fill cap.

I sucked a bit out through the dipstick tube and continued but it had a slight lifter tick at hot idle so I dropped it and put in some 15W40.
Lifter tick went but it still had a slight leak, probably from the infamous external head gasket weep.

Since most on the MB forums suggest either a) HDEO/diesel oil, or b) 15W50, I found a 15W50 diesel oil on sale and threw that in plus a new Mann filter.

1100ppm zinc, 11.1 TBN, and a high viscosity of 19.0cSt @100c, perfect.

The engine runs a bit quieter and smoother as expected but to my surprise the engine temp sits around 5c cooler now, with the only variable being the oil - same ambient temps and roads travelled.
Hwy temp would be around 85-87c with both the 10W30 and 15W40 but it now sits around 82c.
Idle temp in traffic has also dropped from around 92c to 89c.
Bear in mind this car has proper oil pressure and temperature gauges that read accurately and in real time, as opposed to the dummy gauges on my newer cars.

I always thought it was common knowledge that thinner oils help keep things cooler, and that has always been my reasoning for using them, especially in newer, leak free engines.
It seems as though people have documented both ways so I wonder if formulation has more of an effect than viscosity?
I'm interested in other people's experiences with this!
 
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4,710
I do not know about your statement "Thinner oils help keep things cooler."

I believe it is that they transfer heat better, or are supposed to.

Maybe if the thinner oils transferring heat better translates to keeping things cooler is your question..
 
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I've always suspected there was more friction with the use of thinner oils, thus one of the reasons I still use 10w40 in my vehicles even though the oil caps are labeled for 5w30. I seldom see temperatures below 0*F and have never had any problems associated with use of the 10w40. I ran 10w40 in a Ford Escort with a 1.9L. When I quit using the car it had 518K miles and had never had the head off of it. Compression was still 145-155 across all 4 cylinders.
 
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I've always suspected there was more friction with the use of thinner oils, thus one of the reasons I still use 10w40 in my vehicles ...

There is more oil shearing friction with thicker oil, assuming there isn't excessive metal-to-metal contact going on with very thin oils.
 

19jacobob93

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Gold Coast, QL, Australia
I do not know about your statement "Thinner oils help keep things cooler."

I believe it is that they transfer heat better, or are supposed to.

Maybe if the thinner oils transferring heat better translates to keeping things cooler is your question..
Well the theory is as I've read, that thinner oils cause less drag on internal parts, leading to higher engine efficiency and less heat production, and they also dissipate heat better as they flow faster, carrying the heat back to the sump.
According to a few posts I've read on here some people notice consistently lower temperatures after switching to a thinner oil, yet I had exactly the opposite for some reason.
 
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I haven't seen any controlled testing that shows that thinner oil "cools better". Even if it did, it's not going to make any real difference as it would probably be hair splitting (ie, a degree or two difference in oil temp). I would think a thicker (more dense) oil would have more heat capacity, which is one factor involved with how a fluid "cools" parts (ie, transfers heat). But thicker oil also generates more shearing heat, so if thicker/denser oil cools better it's offset somewhat by it's own self generating shearing heat.

I don't know why people worry about this ... when is an engine ever running "too hot" because of the oil viscosity used? Never.
 
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Well the theory is as I've read, that thinner oils cause less drag on internal parts, leading to higher engine efficiency and less heat production, and they also dissipate heat better as they flow faster, carrying the heat back to the sump.

Engines use a positive displacement oil pump, which means 0W-16 flows at the same volumetric rate as 20W-50 does to parts that are feed oil by the oil pump.
 

19jacobob93

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735
Location
Gold Coast, QL, Australia
I've always suspected there was more friction with the use of thinner oils, thus one of the reasons I still use 10w40 in my vehicles even though the oil caps are labeled for 5w30. I seldom see temperatures below 0*F and have never had any problems associated with use of the 10w40. I ran 10w40 in a Ford Escort with a 1.9L. When I quit using the car it had 518K miles and had never had the head off of it. Compression was still 145-155 across all 4 cylinders.
That is some serious mileage. I have never personally seen damage caused by using thicker grades (20w50) although I can imagine it certainly would in colder climates.
When working on my engines I've noticed that regardless of viscosity, the valve trains on all cars are designed to hold a bit of oil up there, especially this M104. There is always a visible film of oil on the cam lobes too which is thick enough to leave a visible mark when I run my fingernail though it. I feel that this boundary lubrication is more than enough to compensate for a few seconds until the flow of oil reaches the top end.
 

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19jacobob93

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735
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Gold Coast, QL, Australia
Engines use a positive displacement oil pump, which means 0W-16 flows at the same volumetric rate as 20W-50 does to parts that are feed oil by the oil pump.
Through the galleries yes, but the oil on the heated internal components (valve train, pistons, cylinder walls) is not under pressure, and this is where viscosity is a factor in the rate at which it returns to the sump to be cooled.
No one is worrying, I just had an contradictory observation that I was hoping someone could explain through experience.
 
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My M111 seems to warm up a tad quicker with 5w30 compared to 10w40 but i only had it in the sump for a month, maybe it is just my imagination. I don't know if that's even possible. 5w30 made the valvetrain quieter on cold starts. In my case, going synthetic and thinner resulted in quieter engines when cold most of the time.
 
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If "thinner" oils, 5W20 truly cool better, I'd still rather have the extra protection, [MOFT] when things get hot, and quieter cold starts during the cold weather the "thicker" oil, 0W30 is providing for me.
 
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1,967
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Well the theory is as I've read, that thinner oils cause less drag on internal parts, leading to higher engine efficiency and less heat production, and they also dissipate heat better as they flow faster, carrying the heat back to the sump.
According to a few posts I've read on here some people notice consistently lower temperatures after switching to a thinner oil, yet I had exactly the opposite for some reason.

Its a false theory as its presented ( and its "presented all over") and mostly done by articles more focused on selling brand than the accuracy of the information.

That being said, there are elements of truth as well- its a very complex non linear subject with a lot of conditions which determine if the application of the theory is true or false.
 
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1,967
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Through the galleries yes, but the oil on the heated internal components (valve train, pistons, cylinder walls) is not under pressure, and this is where viscosity is a factor in the rate at which it returns to the sump to be cooled.
No one is worrying, I just had an contradictory observation that I was hoping someone could explain through experience.

OK, let me ask this because you just ran 2 very different things together that cant be combined.

Are you wanting to know about the actual thermal capacity of the liquid and its transfer rate over the area of travel? ( getting heat from the various parts into the oil by volume)

Or

The hot oil in the sump losing the heat so it can start the process over again?

That's 2 sides of the same coin ( thermal cycle) but even though the same coin- they never touch and have different rules
 
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1,967
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If "thinner" oils, 5W20 truly cool better, I'd still rather have the extra protection, [MOFT] when things get hot, and quieter cold starts during the cold weather the "thicker" oil, 0W30 is providing for me.

Honestly, that's somewhat misrepresented in many articles too.

In a perfect HD/EHD regime when volume and velocity on the supply side is properly specified, that's what controls the MOFT more during thermal increase.

Its true that viscosity affects MOFT but that's fluid specific so it plays more of a part on the top end where the main source of lubrication delivery is flow. In the other areas where the pump is driving the fluid to give a full fluid film, the pump's supply volume "should" overcome any thermal issue related to fluid film thickness and density.

Emphasis on "should"
 
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4,710
One of the basic things I learned is that it is only pressurized oil that can do any lubricating while an engine is running. This discussion seems to be building on that.
 
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35,444
Location
NY
Honestly, that's somewhat misrepresented in many articles too.

In a perfect HD/EHD regime when volume and velocity on the supply side is properly specified, that's what controls the MOFT more during thermal increase.

Its true that viscosity affects MOFT but that's fluid specific so it plays more of a part on the top end where the main source of lubrication delivery is flow. In the other areas where the pump is driving the fluid to give a full fluid film, the pump's supply volume "should" overcome any thermal issue related to fluid film thickness and density.

Emphasis on "should"
Thanks for the info. Good choice of words,"should & perfect." Being old school I come from a world where nothing is perfect, and I like to hedge my bets a little. Especially if things go wrong, or I'm running the engine hard when it's very hot.
 
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1,967
Location
USA
One of the basic things I learned is that it is only pressurized oil that can do any lubricating while an engine is running. This discussion seems to be building on that.

That's not 100% true but it depends on the application and working definition of "pressurized" and are referring to a pump or free flow system

The act of parts coming together creates "pressure" ( a fluid wedge where the fluid acts like a solid) but that's a microscopic thin film but a boundary regardless as opposed to a flooded cavity with a larger film like a crank bearing
 
Messages
1,967
Location
USA
Thanks for the info. Good choice of words,"should & perfect." Being old school I come from a world where nothing is perfect, and I like to hedge my bets a little. Especially if things go wrong, or I'm running the engine hard when it's very hot.

If its designed properly and within tolerance it "will" LOL- its when things get to the edge of those parameters when "should" walks in the door
 
Messages
4,710
That's not 100% true but it depends on the application and working definition of "pressurized" and are referring to a pump or free flow system

The act of parts coming together creates "pressure" ( a fluid wedge where the fluid acts like a solid) but that's a microscopic thin film but a boundary regardless as opposed to a flooded cavity with a larger film like a crank bearing

Is that where splash lubrication comes in? (Top end)
 
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