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Old 26-05-2005   #1
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Post Lets clear up a common misconception (oil viscosity)

I read on many forums about 0w and 5w oils being too thin. I will try to explain it without getting over technical and we'll go from there.

0w-40, 5w-40, 10w-40 and 15w-40 are all the same thickness (14cst) at 100degC.

Centistokes (cst) is the measure of a fluid's resistance to flow (viscosity). It is calculated in terms of the time required for a standard quantity of fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid.

As viscosity varies with temperature, the value is meaningless unless accompanied by the temperature at which it is measured. In the case of oils, viscosity is generally reported in centistokes (cst) and usually measured at 40degC and 100degC.

So, all oils that end in 40 (sae 40) are around 14cst thickness at 100degC.

This applies to all oils that end in the same number, all oils that end in 50 (sae 50) are around 18.5cst at 100degC and all oils that end in 60 (sae 60) are around 24cst at 100degC.

With me so far?

Great!

Now, ALL oils are thicker when cold. Confused? It's true and here is a table to illustrate this.

SAE 40 (straight 40)

Temp degC.........................Viscosity (thickness)


0..........................................2579cst
20..........................................473cst
40..........................................135cst
60..........................................52.2cs t
100........................................ 14cst
120.........................................8.8cst

As you will see, there is penty of viscosity at 0degC, in fact many times more than at 100degC and this is the problem especially in cold weather, can the oil flow quick enough to protect vital engine parts at start up. Not really!

So, given that an sae 40 is 14cst at 100degC which is adequate viscosity to protect the engine, and much thicker when cold, how can a 0w oil be too thin?

Well, it can't is the truth.

The clever part (thanks to synthetics) is that thin base oils can be used so that start up viscosity (on say a 5w-40 at 0degC) is reduced to around 800cst and this obviously gives much better flow than a monograde sae 40 (2579cst as quoted above).

So, how does this happen, well as explained at the beginning, it's all about temperature, yes a thin base oil is still thicker when cold than at 100degC but the clever stuff (due to synthetics again) is that the chemists are able to build these oils out of molecules that do not thin to less than 14cst at 100degC!

What are the parameters for our recommendations?

Well, we always talk about good cold start protection, by this we mean flow so a 5w will flow better than a 10w and so on. This is why we recommend 5w or 10w as the thickest you want to use except in exceptional circumstances. Flow is critical to protect the engine from wear!

We also talk about oil temps, mods and what the car is used for. This is related to the second number xw-(XX) as there may be issues with oil temperatures causing the oil to be too thin and therefore the possibility of metal to metal contact.

This is difficult to explain but, if for example your oil temp does not exceed 120degC at any time then a good "shear stable" sae 40 is perfectly capable of giving protection.

"Shear stability" is important here because if the oil shears it thins and that's not good!

However, if you are seeing temperatures in excess of 120degC due to mods and track use etc then there is a strong argument to using an sae 50 as it will have more viscosity at these excessive temperatures.

There are trade offs here. Thicker oils cause more friction and therefore more heat and they waste power and affect fuel consumption so it's always best to use the thinnest oil (i.e. second number) that you can get away with and still maintain oil pressure.

There is more but this post is too long already so lets keep it to basics.

Cheers
Simon
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Old 26-05-2005   #2
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Re: Lets clear up a common misconception (oil viscosity)

Great post as usual Simon

If only the Germans had made the right decision on oils for their Panzers they probably would have won the war....
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Old 26-05-2005   #3
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Re: Lets clear up a common misconception (oil viscosity)

So to conclude, don't worry about it and just stick to recommended types.

Excellent, thanks. Great post
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Old 26-05-2005   #4
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Re: Lets clear up a common misconception (oil viscosity)

why some posts got Simon on them and others Guy?
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Old 26-05-2005   #5
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Re: Lets clear up a common misconception (oil viscosity)

Easy.

Simon is my brother, together we post under the name of Oilman to save confusion.

We post on over 100 forums so it is a two man job

I am the one with the Fiat though

Cheers

Guy.
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Old 26-05-2005   #6
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Re: Lets clear up a common misconception (oil viscosity)

Quote Originally Posted by oilman

Simon is my brother, together we post under the name of Oilman to save confusion.
that didnt work then coz i was well confused
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Old 26-05-2005   #7
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Re: Lets clear up a common misconception (oil viscosity)

Quote Originally Posted by oilman
We post on over 100 forums so it is a two man job
I have to ask. Why?
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Old 26-05-2005   #8
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Re: Lets clear up a common misconception (oil viscosity)

Quote Originally Posted by dave
that didnt work then coz i was well confused
OK.

My Father had two sons.

Son number 1 = Simon (he is older)
Son number 2 = Guy ( he is a little bit younger)

We, both post under the same name.

Did you get it that time?

Cheers

Guy.
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Old 26-05-2005   #9
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Re: Lets clear up a common misconception (oil viscosity)

Quote Originally Posted by Steve C
I have to ask. Why?
Thats what I do for a living.

I am here because I have a fiat though.

Cheers.

Guy.
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Old 26-05-2005   #10
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Re: Lets clear up a common misconception (oil viscosity)

You post on forums for a living? Kool job, care to employ me? I spend a lot of time on forums so I have excellent qualifications
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Old 26-05-2005   #11
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Re: Lets clear up a common misconception (oil viscosity)

It is a strange job.

We have deals with many internet car forums as their advisors/suppliers of oil.

http://www.opieoils.co.uk/lubricants.htm go and have a look.

Hope this clears it up.

Cheers

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Old 26-05-2005   #12
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Re: Lets clear up a common misconception (oil viscosity)

Quote Originally Posted by oilman
OK.

My Father had two sons.

Son number 1 = Simon (he is older)
Son number 2 = Guy ( he is a little bit younger)

We, both post under the same name.

Did you get it that time?

Cheers

Guy.

That's frickin funny
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Old 26-05-2005   #13
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Re: Lets clear up a common misconception (oil viscosity)

.. so a 0w40 oil is thinner when cold than a 15w40 then?
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Old 26-05-2005   #14
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Re: Lets clear up a common misconception (oil viscosity)

Thats really weird!!!!

My name is Simon and my younger is brother called Guy. Spooky :P
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Old 26-05-2005   #15
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Re: Lets clear up a common misconception (oil viscosity)

To answer the post above by fixitagaintomorrow, "Yes, but only at the low end. The first number in an oil's grade is a cold weather index. Lower numbers pour easier in cold temperatures. The second number is an index of heat resistance. In this case they were both 40, so no difference."

Thanks, Guy and Simon. I sold oils and parts for quite some time. The odd bit of that is that I was working for the tax free shop of the US Forces (like the UK NAFFI) so I had to deal with both USA and Euro vehicles here in Belgium. My former Clio had synthetic 5W30 from the first oil change until the end of the warranty. Then I changed to 0W30. I got over 201,000 miles out of it in just under six years. A lot of US cars tell the owners to change the oil each 3000 miles. OK, if it's in warranty, do it. If not, use synthetic and go for between 9 and 12 thousand...

Here's one that I can't figure out. Most US market cars since 1995 say to use 5W30. Honda and Ford in the US are using 5W20. Here in Europe, the recommended grades are MUCH higher. My Idea 1.9 says to use 10W40 and 5W40 only in the winter. Many other cars ask for 15W40 here. Well, after about 2500 miles, the factory oil came out of the Idea and it was replaced by Mobil 1 0W40 and the engine has not yet exploded!!! Given that petrol and diesel in Europe are two to three times the American prices, why are the carmakers still saying to use these old-fashioned heavy oils? The 0W30 in my Clio was not on Renault's approved list for that engine and year but the engine ran perfectly on it for a very long time. What's Fiat's position on this? Would my Idea do even better with 5W30 or even 0W30? Yes the Clio was also a diesel, but without turbo.

On oil additives... Do not do this except as follows. Typical additives are for people who don't know any better. Don't use them. If you use synthetic oil, use a synthetic additive. Pouring a quart of Slick 50, STP, or any other mineral based oil additive in to synthetic oil is DDUUMMBB!! Use only a full synthetic additive in your expensive full synthetic oil for maximum protection and fuel savings. If you have a diesel, make sure what you are wanting to put in your engine is up to the task as temperatures and pressures inside of a diesel are far greater than in a petrol engine. I use X-1R, but I have no idea if it is sold in the UK or not. If what you're considering doesn't say "diesel approved" or similar on the packaging, don't use it.

Hope I've enlightened and even entertained at least a few of you with this. Please ask if you have more questions.
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Last edited by raton_laveur; 26-05-2005 at 22:26.
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