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Old 14-10-2018, 16:52   #1
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Oil Temp. vs Viscosity/Density WTF?

Noticed a few posts where folk mention using multigrade oils in raw water cooled engines. Did some checking on viscosity/density measurements of various oils and all multi and mono grade oils show similar decreasing graphlines with rising temperatures. This seems to contradict the idea that multigrade oils contain additives that increase viscosity/density as temps rise. Is it the case then, that additives merely change the rate of decrease in visc./dens. ? There seems to be very little difference in the visc./dens. of a 30w mono and a 15w-40 multi oil in the 50c-60c operating range of a raw water cooled diesel
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Old 14-10-2018, 17:35   #2
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Re: Oil Temp. vs Viscosity/Density WTF?

We used multi grade oils in our trucks to make them easier to start in cold weather not for increasing viscosity as temps rise ! single grade/wt., would decrease as the temp came up, after x amount of miles ! as would the multi. !
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Old 14-10-2018, 17:47   #3
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Re: Oil Temp. vs Viscosity/Density WTF?

You are correct that the additives "merely" decrease the reduction in viscosity as the temperature increases. The simplistic way to think of it, a 10W30 oil has similar viscosity to a 10W oil at cold starting temperatures and similar viscosity to a 30W oil at hot operating temperatures.
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Old 14-10-2018, 18:46   #4
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Oil Temp. vs Viscosity/Density WTF?

Quote:
Originally Posted by charliehows View Post
Noticed a few posts where folk mention using multigrade oils in raw water cooled engines. Did some checking on viscosity/density measurements of various oils and all multi and mono grade oils show similar decreasing graphlines with rising temperatures. This seems to contradict the idea that multigrade oils contain additives that increase viscosity/density as temps rise. Is it the case then, that additives merely change the rate of decrease in visc./dens. ? There seems to be very little difference in the visc./dens. of a 30w mono and a 15w-40 multi oil in the 50c-60c operating range of a raw water cooled diesel


Yes, you are correct the Viscosity Improver additive can only resist changes in viscosity, it canít actually thicken or thin oil, it resists changes.
Itís also usually the first additive package to break down too, but we shouldnít really cause oil break down in our little motors.
Now the turbo guys who believe you need to run at least 80%, now they may break down oil I guess, maybe.
A 15w-40 oil is not going to hurt any older design Diesel, whether raw water cooled or not, a question is for the ones used in very warm weather that most of us cruise in, does it do any good?
I think not, but that is just my belief.
If I were to buy into if you will multi viscosity oil, Iíd go all the way to Rotella T6 5W-40 oil, an excellent oil, but is it needed or will it make any difference? I think not.
But thatís just my opinion.

I used to run T6, partially so I could run only one oil in my boat, outboard and generator etc, just buy and store one oil.
I still use T6 in our little turbo Miata and the Wifeís CTS-V, which is in storage, so I guess Iím not using anything in it.

I went with straight 30W under the assumption that itís likely to be available pretty much anywhere and I hope to cruise off of the beaten path.
In my little motor, a 4JHE itís not enough money difference to matter, but some measure oil changes in gallons, not quarts, so for them itís a different story.

My engine manual only specs straight weight oil, no multi viscosity oils, but I feel sure that is more to do with the age of the motor and not the oil. Iím sure a 15W-40 even though itís not called out in the manual would be fine.
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Old 15-10-2018, 11:21   #5
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Re: Oil Temp. vs Viscosity/Density WTF?

Quote:
Originally Posted by charliehows View Post
Noticed a few posts where folk mention using multigrade oils in raw water cooled engines. Did some checking on viscosity/density measurements of various oils and all multi and mono grade oils show similar decreasing graphlines with rising temperatures. This seems to contradict the idea that multigrade oils contain additives that increase viscosity/density as temps rise. Is it the case then, that additives merely change the rate of decrease in visc./dens. ? There seems to be very little difference in the visc./dens. of a 30w mono and a 15w-40 multi oil in the 50c-60c operating range of a raw water cooled diesel
It has nothing if little to do with maintaining viscosity. It is the lubricating ability that stays relatively constant through the range of viscosity so I've been led to believe.
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Old 15-10-2018, 11:21   #6
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Re: Oil Temp. vs Viscosity/Density WTF?

Is it the case then, that additives merely change the rate of decrease in visc./dens. ? yes.
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Old 15-10-2018, 11:36   #7
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Re: Oil Temp. vs Viscosity/Density WTF?

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Is it the case then, that additives merely change the rate of decrease in visc./dens. ? yes.
yes

you can't beat the laws of thermodynamics

"In general, the viscosity of a simple liquid decreases with increasing temperature."
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Old 15-10-2018, 15:12   #8
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Re: Oil Temp. vs Viscosity/Density WTF?

so I neednt be quite so fussy about the engine oil but the little yanmar has another convenient characteristic - at least, mine does - in that the gearbox also specifies 30w mono, which begs the question "is it ok to use a multi-grade (eg 15w-40) in the gearbox?" - any obvious reasons why not?
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Old 15-10-2018, 15:16   #9
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Re: Oil Temp. vs Viscosity/Density WTF?

It surely wouldnít hurt, but why risk it?
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Old 15-10-2018, 15:16   #10
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Re: Oil Temp. vs Viscosity/Density WTF?

Timely thread, this question has been occupying my head for some days now; looking forward to all the responses.
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Old 15-10-2018, 15:29   #11
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Re: Oil Temp. vs Viscosity/Density WTF?

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It surely wouldnít hurt, but why risk it?
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Old 15-10-2018, 15:36   #12
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Re: Oil Temp. vs Viscosity/Density WTF?

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It surely wouldnít hurt, but why risk it?
fair enough - it only takes about a litre...
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Old 15-10-2018, 15:42   #13
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Re: Oil Temp. vs Viscosity/Density WTF?

My transmission, (Paragon SAO) calls for an old spec auto trans fluid but I use 30wt engine oil. It works fine. I notice the engine and trans run at the same oil temp too, 185 deg F.
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Old 15-10-2018, 15:43   #14
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Re: Oil Temp. vs Viscosity/Density WTF?

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It has nothing if little to do with maintaining viscosity. It is the lubricating ability that stays relatively constant through the range of viscosity so I've been led to believe.
How is the lubricating ability measured? In other words, what are the units of this factor?

I have been reading oil technical specifications for a few nights now and I am slowly coming to grips with some of the terms and how they are measured. I getting a reasonable understanding of the various ways of quantifying viscosity but I would love to fastback an understanding of lubricating ability and the method of quantifying it.
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Old 16-10-2018, 09:18   #15
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Re: Oil Temp. vs Viscosity/Density WTF?

Viscosity is measured in C's or Centi Stock
Viscosity really means: resistance to flow.
All oils have two major factors to consider namely
SAE spec and API specs
SAE : Society of Automotive Engineering which refers to the oil's viscosity e.g. 10w40 etc.
API: American Petroleum Institution that refers to the additive package "cocktail" of the oil.
Yes, it is important to consider the operating temperature of a given component eg engine , gearbox etc. At the same time the ambient temperature where the component works should not be under estimated.
Also...as a rule of thumb, older engines that reach the final stage of their life cycle, respond better to mono grade SAE e.g. 30. It helps in cold start situation and builds oil pressure quicker during start up yet gives you the protection at the high end.
Be careful about using engine oil in gearbox, refer very closely to the OEM recommendation. Gearboxes do not require any TBN (total base number) that engine oil must have at the same time gearboxes require certain EP (extreme pressure) arditives in order to manage the load on the gears.... The same EP additives can be detrimental to your engine's life.
In summary: stick to the manufacturer's recommendation, stick to well known respectable brands of oil and try as much as possible to use the same brand of oil throughout the life of the components.
Do not save on oils and/or filters...
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