Are 5w And 0w Oils Too Thin?

[oilman 7]

I read on many forums about 0w and 5w oils being too thin.

 

0w-40, 5w-40, 10w-40 and 15w-40 are all the same thickness (14 centistokes) 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 plenty 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.

 

I hope this helps explain a bit.

 

Cheers

 

Guy.

[pee vee 1]

Hi Guy.

thanks for your post, informative as always!!

 

can you tell me. at what point (in degrees C) does engine oil start to become

in-efficient?

 

 

thanks!

[matty_gti 20]

Hi Guy.

thanks for your post, informative as always!!

 

can you tell me. at what point (in degrees C) does engine oil start to become

in-efficient?

thanks!

 

Read it again...... , its all in there.

[matt350cos 0]

Thanks Guy you have cleared some things up for me i am running oil that is wrong when cold

 

Thankyou

[KRISKARRERA 2 1 Cars]

Sorry I can't read all that tonight, my head hurts .

Am I right in saying a 5w40 or 0w40 is no thinner than a 10w40 once the engine has warmed up therefore this thing about 5w and 0w oils being too thin is crap?

[Mikey G 3]

Sorry I can't read all that tonight, my head hurts .

Am I right in saying a 5w40 or 0w40 is no thinner than a 10w40 once the engine has warmed up therefore this thing about 5w and 0w oils being too thin is crap?

 

Yes thats right as the 40 is the common thickness when warm, the first number is its cold properties.

 

Ive read a few write up's from Guy now and have learnt a lot from them, thank you.

Edited June 16, 2007 by Mikey G

[M_R_205 5 1 Cars]

useful info as always thanks guy, i now think i will replace my 10-40 with 5-40 insted

[KRISKARRERA 2 1 Cars]

I change my oil every 1000 miles / 3 months - using a semi 10w40. Would I be better off upgrading to 5w40 and changing the oil half as much?

[niklas 1]

I change my oil every 1000 miles / 3 months - using a semi 10w40. Would I be better off upgrading to 5w40 and changing the oil half as much?

 

As long as you're changing that often, including filter, it does barely matter which oil you use!

Unless you have special needs of course, like racing (and extreme oil temp's) or a lot of cold starts..

But whether it's 0w40, 5w40, 10w40, castrol or tesco (do they sell oil? ) doesn't really matter! Just get decent filters!

[KRISKARRERA 2 1 Cars]

Well I'm only changing the filter every other oil change.

[gti-si 0]

castrol or tesco (do they sell oil? ) doesn't really matter! Just get decent filters!

 

Tesco do sell oil, fairly cheap too! Gay, though, they only sell it in 2L and 4L...So for an oil change which is 5L it's a pain in the arse

[niklas 1]

Well I'm only changing the filter every other oil change.

 

I would change the filters at every oil change as well. Maybe I'm exaggerating but it doesn't cost much money and it won't hurt..

 

Tesco do sell oil, fairly cheap too! Gay, though, they only sell it in 2L and 4L...So for an oil change which is 5L it's a pain in the arse

 

For every 5th oil change you'll even up though!

[Beastie 1]

Well I'm only changing the filter every other oil change.

 

And I thought *I* was a tight git ;P

[KRISKARRERA 2 1 Cars]

LOL but that's a bit OTT isn't it? Imean that would mean an oil filter is only on the car for around 1000 miles, yet the interval for many cars is now 12000miles.

[hengti 2]

I change my oil every 1000 miles / 3 months - using a semi 10w40. Would I be better off upgrading to 5w40 and changing the oil half as much?

 

that must be costing a fortune!

 

admittedly mine was a daily driver, but i only ever changed the oil (Duckhams Q 10/40 - with filter) every 6,000 miles in mine (1.9 8v 205); i thrashed the knackers off it for 8 years and eventually broke it for parts with the original engine still in it. it still had excellent (indicated) oil pressure and good compression at 211,000 miles..

 

apparently, Elf Excellium 5/40 is what's required of the replacement (172 Cup); don't know how much it costs for a tub yet, but it sounds expensive..

[niklas 1]

I change my oil every 1000 miles / 3 months - using a semi 10w40. Would I be better off upgrading to 5w40 and changing the oil half as much?

 

Ahh my conversion was a fair bit out. 1000 miles is maybe a bit overdoing it. I would say 6000 is caring, 9000 slentrian and above is plain hatred

[Pugnut 9 1 Cars]

when we sold my wifes saxo it had done 80k with one oil change at at 12k and one round about 60. went like stink for a 1L , still going strong when it was sold . being a 1L it was thrashed to within an inch of its life on a daily basis too.

 

we've had the wifes 307 since 15k and its now nearing 70 and it has only had an oil change at its 60k service.

 

on the other hand my bmw 323 ci has had 3 oil changes in 3 years with mobil 1 and i've done 4k !!

 

i've got my priorities right!

[KRISKARRERA 2 1 Cars]

Mine does alot of cold starts and short journies. Plus I buy the oil in bulk - about £50 delivered for 25 litres of 10w40 semi synthetic.

[Anulfo 22]

Tesco's do great fuel,i recommend the Tesco 99 octane fuel!!!

As for tesco oil i don't know.If they do then probably like so many of these oils they come from the same refineries so you are probably buying the same oil anyway.Im sure Millers refine their own oils though?

As for type of oil one so called expert on pugs told me that these engines should have 10w40 semi synth as the fully synth is far too advanced as these are old engines designed years ago!!!

[gsidave 3]

where i work at a pug main dealer we use 5/40 fully synthetic on every car there but i asked our master tech about the oil for my 1.6 gti he said use a semi 10/40 as the xu is an old engine with poor tolerances compaired to modern cars which are expected to do 12k plus, so thats what i used on the advice from our master tech

[Rom 0]

Sorry to bring back an old topic.

 

But after the running in,i swapped my rebuilt Mi to 0/40 Mobil 1 Fully (what i get for free from work).

But speaking to a senior tech, he said even though my engine is rebuilt, its not supposed to run such good oil. Saying my scraper rings will have trouble clearign the oil etc. And possibly doing more damage than good.

 

I always thought of the old engine / thicker oil debate as being it would simply leak out easier. Now mine holds the Mobil 1 in. But should it be using it ? I understand the hot viscosity is the same, and im getting better cold protection etc.

Should i swap to 10/40 semi instead ?

[large 33]

I just keep topping mine up About a 1 pint a week . On the plus side the road infront of my house looks A1 in the wet

[oilman 7]

0w-40 should be fine but a 5w-40 may be more suited.

 

Whether you go you 10w-40 semi is a question of cost and what you are using the car for. If it's modded or used on track then semi is not the best choice as it's not really up to it.

 

Cheers

[KRISKARRERA 2 1 Cars]

Do you think then I'd be better off paying extra for a synthetic 5w40 and changing the oil less often?

[Furiousd 0]

Just reading this and I thought I'd point out that my last oil change was to 5w40 fully synthetic as thats all the garage I went to had. That was a about 2 months ago and the car has had a pretty bad oil leak since (Didn't leak before)

 

Could this be down to the engine not holding the thinner oil? or possibly an ill fitted oil filter?

 

The problems gone from a niggle to a major leak in the last weak, nearly ran the oil dry to the point I could see the oil pressure drop when pulling away from t junctions

 

Checking the oil every day now and topping it up until I can get it booked into a garage to get the leak fixed