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Clarification on a common misconception (oil viscosity)
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Old 27 Apr 2005, 10:10 AM   #1 (permalink)
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Clarification on a common misconception (oil viscosity)

I wrote this recently for a thread on Scoobynet and felt it would also be of interest here.

I see it mentioned so many times, I can't help but respond as the facts often quoted on these boards are completely wrong!

Why is it that in some Subaru manuals warn against the use of 5w-30?

This is because the use of sae 30 is not recommended and in fact has nothing to do with the "w" number which is 5.

This leads to comments like:

"You must not use 5w oils"

"5w oils are too thin"

"Use 10w-40 but not 5w-40"

Some facts need to be straightened out here and hopefully I can do this without losing those that are interested. Others will undoubtably pick parts of the post, quote them and then pick a fight with me turning this into a ridiculously confusing thread - Let's hope not.

So, given that sae 30 is not recommended what are the reasons?

SAE 30 must be in Subaru's opinion too thin at higher temperatures (100degC) for the engine to give the adequate metal to metal contact protection that the engine requires over a period of time as all oils shear (lose viscosity) with use.

I do not often recommended to use of sae 30 oils for any scoobies new or old although it does appear that some of the newer models do not exclude the use of 5w-30 or 10w-30 and some technical databases list these grades.

I usually tend to recommend the use of sae 40 and sae 50 as the best options depending on the age and specification of the car.

So what is the difference between a 5w-40 and a 10w-40 @100degC?

Basically NONE!

SAE determines the viscosity (+/-) that the oil needs to be at 100 degC and these need to be met in tests to give the oil it's API rating (xw-xx or xxw-xx).

They are as follows: (within a margin)

SAE 30 = 11.00cst
SAE 40 = 14.00cst
SAE 50 = 18.50cst
SAE 60 = 24.00cst

Compare these SAE 40 oils at 100degC (0w, 5w, 10w, 15w)

Motul 300V 5w-40.................Viscosity = 13.80cst
Motul 300V 10w-40................Viscosity = 14.00cst
Silkolene PRO S 5w-40............Viscosity = 14.89cst
Motul 8100 X-cess 5w-40........Viscosity = 14.00cst
Fuchs Titan Supersyn 5w-40....Viscosity = 13.60cst
Castrol Performance 10w-40.....Viscosity = 14.50cst
Silkolene XTR 10w-40 (semi).....Viscosity = 14.70cst
Mobil 1 0w-40........................Viscosity = 14.30cst
Motul 8100 0w-40...................Viscosity = 13.30cst
Silkolene Turbolene D 15w-40....Viscosity = 14.40cst
Total Quartz 15w-40...............Viscosity = 14.50cst

These figures are not by chance and include synthetics, semi-synthetics and mineral oils they are all in the range to be labelled a Xw or XXw-40 multigrade oil.

So, what's the "W" number all about then?

It stands for "winter" not "weight" as often confused and called on many U.S. articles on oil!

This number is the "cold crank" viscosity and nothing to do with the oil viscosity when the engine is up to temperture. These numbers are related to the oils ability to operate in cold temperatures. 0w oils were originally designed to operate in arctic climates that's why they operate at ferrous monkey endangering temperatures of -35degC and below!

The benefits of the lower viscosity oils (0w and 5w) is that they flow more easily and quickly when cold and therefore protect the engine better on cold start when 80% of the engine wear occurs.

So, yes it's true an sae 40 is an sae 40 when hot whether its a 0w, 5w, 10w, 15w or whatever and that's a fact. The same goes for 20's, 30's, 50's, 60's and so on.

Hope this helps.

Cheers
Simon
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Old 27 Apr 2005, 10:20 AM   #2 (permalink)
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excellent read that simon. Thanks for taking the time to post up for the dot org users
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Old 27 Apr 2005, 10:47 AM   #3 (permalink)
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Oil

Simon,

Good read mate,

So to summarise in really BASIC cooking style terms then...

the first number w5, w10 w 15 etc is a value for how runny the oils is when cold (higher the number the thicker)?

and the second number 30, 40, 50 is how runny the oil is at 100C (higher number the thicker)?

Jon
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Old 27 Apr 2005, 11:23 AM   #4 (permalink)
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Yep, 5* summary there.

The higher number is more important when the engine is stressed.

Cheers
Simon
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Old 27 Apr 2005, 03:08 PM   #5 (permalink)
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Simon

So on that theory Silkolene Pro S 5w 40 is best suited out of the list above??????
Good cold engine qualitys and Best CST at 100deg C

Oh that's what I've ordered
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Old 27 Apr 2005, 03:15 PM   #6 (permalink)
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No offence to all the others listed there (and we sell them all) we like the Silkolene and consider it as the best. Motul 300V comes a very close second though!

Cheers
Simon
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Old 27 Apr 2005, 03:35 PM   #7 (permalink)
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Never tried the Silkolene, But I used to use the MOTUL 300V in my Integrale, it was the only thing that kept the oil pressure up when hot.
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Old 28 Apr 2005, 07:34 AM   #8 (permalink)
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from an engineering point of view (rather than pure sales) and admittedly mainly on competition/higher power cars (scooby's and evo's)- we have found "issues" with Silkolene generally that were not present with other oils/products.......................

personal preference i would only use MOTUL or MILLERS competition products and our advise/stock is aligned with this also.....................................

* from an "OLD" silkolene agent
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Old 12 May 2005, 03:18 PM   #9 (permalink)
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Prey do tell, what issue were those then.

Purely from a sales and satified Customers point of view, 12 years here and no issues to speak of.

Sweeping statements like this are unfair and of no use to anyone. As an ex agent do you have an axe to grind :wink:

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Simon
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Old 13 May 2005, 09:14 PM   #10 (permalink)
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Simon, i gave up my agency as i did not want to align myself with a product that i had lost faith in...........................

i don't sell or use any product that i do not have 100% faith in, engines are too expensive as it is without worrying about cosumables that potentially cost £1000's..........


as for 'issues', we had a couple of engine failures mainly on gpN turbo engines with a/lag, etc. (high heat, extreme use!!) and could find little or no reson for these, after discussions with a very helpful small and focused oil company we used a different formulation/grade of oil and never had another under the same conditions...........coincidence, maybe, but at the end of the day the customer would never go back and to be honest neither would we!!
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Old 13 May 2005, 10:49 PM   #11 (permalink)
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Who's to know what actually happened, oil analysis would have been the sure fired way to prove whether the oil was in good shape or not.

I'm not having a go, just trying to be constructive.

We too only sell products that we have absolute faith in. Our profits on one can of oil to another do not alter it's a fixed pence per litre margin.

Unlike you, having sold Silkolene's products for more than 10 years we have utmost faith in them and have not had cause to do otherwise.

We can always have the oil independently analysed, it costs very little but tells you an awful lot about what actually happened.

Thanks for the comments.

Cheers
Simon
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Old 14 May 2005, 07:37 AM   #12 (permalink)
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the oils WAS analysed, however it was sent to Silkolenes's own chemists and the results suprisingly enough took no blame.................
we build engines (we don't blend oil) and as such take scientific advice to help with diagnostics, however, our feelings on these particular engine failures were that it was oil related because of the damage (the reason for the oil analysis, never happened on the same spec engines before changing brands an never happened after!!

make of that what you will, its my experience..............................
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Old 16 May 2005, 04:57 PM   #13 (permalink)
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Shell Ultra?

Hi,

Following this thread I thought I better check what the subaru grage used to fill my05 wrx on its first proper service:- Shell Ultra Helix 5w 30???? So I checked the manual and 5w 30 is recomended for turbo models? Is this right? comments please biggining to panic now

Cheers Jon
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Old 16 May 2005, 06:21 PM   #14 (permalink)
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5w-30 or 5w-40 is fine.

Cheers
Simon
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Old 17 May 2005, 09:38 AM   #15 (permalink)
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helix

Thanks Simon,

When I purchased some last night to top up I found that shell helix ultra is actually 5w 40, the guy in service must have wrote it down wrong

Jon
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