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

 

cool interesting read

 

 

Went straight over my head

Cheers
Simon

 

The problem is, that a 5W40 is thicker when hot, than a 0W40.

It seems obvious if u go off the above facts, that we should all be using 0W40. Indeed, 0W is the best grade for any engine when it's cold - as its almost impossible to have an oil too thin on startup.

I have done a lot of tests with diff oils measuring temp against pressure. A CVH should run 40psi of pressure when oil is hot, at 3000rpm. My own engine has covered less than 10k since a very thorough rebuild. It does however, have quite large bearing/crank clearances as common with race engines - but these are still within tolerance for a new engine.

With Mobil 0W40 - i can get no more than 32psi at any rpm. With Valvoline racing 5W50 i get around 38psi, and with millers 10W60 i get 40psi. My concern with millers is the cold start protection, but being an ester synthetic it should be good.

It should be noted, that pressure doesnt lubricate, flow does. However, 32psi is a little low to prevent crank/bearing contact in extreme conditions. My aim is to find the thinest oil which give the correct 40psi/3000rpm at 200F. Going to try some silkolene next.

 

Top Post.

However. what is the actual diffrence between Magnatex, Valvoline, etc, appart from the synthetic, semi, etc and the Obvious viscosity? I have a standard 1.4 cvh at the moment and was wondering whether it is worth paying more for these oil's or not? what does everyone else use? Is it purely marketing rubbish?

Also i asume a (from the rest of the post) that a 0w 40 would b the best oil for my engine, or would it be 2 thin on startup / Not cope with full temp properly? I fully understand the rest of the post i just would like to clarify this.

I will stop here as i am sure you get the general idea of this post.

Thank's once again for this post on this usefull, yet much misunderstood, topic.

 

Went over my head too. I know from experiance of actually testing oils to distruction that fully synthetic thins out too much at high rpm and operating temperatures.

Semi synthetic doesn't do it as bad.

How do i know this? Cos at 140mph my tappets are screaming, the bottom end sounds tinny, and when i stop and check the oil its like water and has absolutely no lubricating properties to it.

All this technical explanation doesn't change my view on oils at all, i go by what i've seen.

p.s
Dunno about anyone else, but the only time my engine see's 40 + 100 degrees is about 2 seconds after starting up. Then it runs at 150-200 at normal temp. These oil gurus who make this shit should actually get out of the lab and drive the fucking cars!

 

Snowy Astra - you're actualy destroing your turbocharger apart form averything else by usin non-synthethic oils. I already asked you once on similar occasion - did you use proper synthethic targeted at performance/sports cars? In motorsport nowedays there are only synthethic oils even in old archaic constrution cars. On very oil demanding rallycross group A air-cooled Fiat 126p with which I used to help during events we used Castrol TWS 10W60 with no problem. Even though on this car cooling dificency is so big that it easily sees temperatures like 140-150 degrees and starts to "knock" at the end of each race.

 

I'm not advocating that 0w-40 is the solution to all engines as they vary so much due to modding and set-up.

What I am illustrating here is that an sae 40 (be it 0w-40, 5w-40, 10w-40 or 15w-40) is still an sae 40 and at 100degC thins to 14cst.

I am also stating that an sae 40 is good for an oil temp of 120degC and the switch to an sae 50 within this temperature range is wasting power at the wheels.

Sure if you are seeing oil temps above 120degC then an sae 50 is certainly a better choice for a higher level of protection in extreme cases.

With regards to oil pressure, I had a long discussion with a technical expert recently and he made some valid points which were as follows.

Oil pressure is initially defined by engine design, and it doesn’t have to be high.

The mighty Cosworth DFV V-8 which dominated F1 for 15+ years was (and still is) happy with about 45PSI. The Le Mans-winning Bentleys of the 1920s were designed to run on 5-10PSI once the oil was hot.

Obviously, an engine which would always give massive oil pressure would be one built with microscopic bearing clearances, so there would be nowhere for the oil to go! 150PSI, but total seizure in a few seconds!

On the other hand, oil pressure significantly lower than the spec. minimum means that bearings ‘at the end of the queue’ for oil feed get a low flow rate, which is not desirable.

Even so, a good flow rate with a wear-resistant low-viscosity oil will improve cooling and be better for the engine than another 10PSI with a thicker oil. (….and less chance of cavitation problems.

With regards to marketing guff, I think that some of my earlier posts concerning shear stability say it all. It's not guff, it's fact. All oils shear (lose viscosity) with use and this means lower protection at higher temperatures. I've explained this before but it's probably on page 1000 by now so I'll post the explanation again.

Viscosity Index Improvers.

An oils viscosity will decrease as the engine temperature rises. Viscosity Index Improvers are added to reduce this thinning. They are a key addative in the production of multigrade oils.

VI Improvers are heat sensitive long chain, high molecular weight polymers that increase the relative viscosity of the oil at high temperatures. They work like springs, coiled at low temperatures and uncoiling at high temperatures. This makes the molecules larger (at high temps) which increases internal resistance within the thinning oil. They in effect "fight back" against the viscosity loss in the oil.

"Shearing"

The long chain molecules in VI Improvers are prone to "shearing" with use which reduces their ability to prevent the oil from losing viscosity. This "shearing" occurs when shear stress ruptures the long chain molecules and converts them to shorter, lower weight molecules. The shorter, lower weight molecules offer less resistance to flow and their ability to maintain viscosity is reduced.

This shearing not only reduces the viscosity of the oil but can cause piston ring sticking (due to deposits), increased oil consumption and increased engine wear.

Like basestock quality, VI Improvers also vary in quality. The best quality ones are normally found in synthetic oils (Group IV - PAO / Group V - Esters) and it is important to understand that the less of these in the oil the better the oil will stay in grade.

Which oils require more VI Improvers?

There are two scenarios where large amounts of these polymers are required as a rule.

Firstly in "wide viscosity" multigrades. By this I mean that the difference between the lower "W" number and the higher number is large for example 5w-50 (diff 45) and 10w-60 (diff 50) are what is termed as "wide viscosity" oils.

Narrow viscosity oils like 0w-30 (diff 30) or 5w-40 (diff 35) require far less VI Improvers and therefore are less prone to "shearing".

Secondly, mineral and hydrocracked (petroleum synthetic oils) require more VI Improvers than proper PAO/Ester (Group IV or V) synthetic oils as they are less thermally stable to begin with and this is due to the non-uniform molecules in petroleum oils as opposed to the uniformity of synthetics built in laboratories by chemists.

It is a fact that some synthetics require little or no VI Improvers to work as a multigrade due to their superior thermal stability.

At the end of the day it's your car and your money but without the facts I can't see how you can possibly make an educated decision.

Cheers
Simon

 

Cheers, i think that's cleared up all my questions.