Are 5w and 0w oils too thin?

[oilman 0]

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.

[craigowl 0]

Very interesting and educational. 8)

Thanks to our oilman. :thumb right:

[vardo 0]

8) great info well in

[oilman 0]

You're welcome 8)

[ZippyVR6 0]

Mr Oilman,

 

I hears somewhere that modern oils have plastersizers in them which increase teh bulk of oil when hot. Is this true and how do they work?

[oilman 0]

They are in fact polymers that are heat sensitive and uncurl with heat. These are contained in all multigrade oils of any quality. Synthetics actually contain less of these than mineral oils.

 

These polymers are called VI Improvers

 

What is VI Improver?

 

As a lubricant basestock is subjected to increasing temperatures it tends to lose its viscosity. In other words, it thins out. This leads to decreased engine protection and a higher likelihood of metal to metal contact. Therefore, if this viscosity loss can be minimized, the probability of unnecessary engine wear will be reduced.

 

VI improvers are polymers that expand and contract with changes in temperature. At low temperatures they are very compact and affect the viscosity of a lubricant very little. But, at high temperatures these polymers "expand" into much larger long-chain polymers which significantly increase the viscosity of their host lubricant.

 

So, as the basestock loses viscosity with increases in temperature, VI improvers “fight back” against the viscosity drop by increasing their size. The higher the molecular weight of the polymers used, the better the power of "thickening" within the lubricant. Unfortunately, an increase in molecular weight also leads to an inherent instability of the polymers themselves. They become much more prone to shearing within an engine.

 

As these polymers are sheared back to lower molecular weight molecules, their effectiveness as a VI improver decreases. Unfortunately, because petroleum basestocks are so prone to viscosity loss at high temperatures, high molecular weight polymers must be used. Since these polymers are more prone to shearing than lower molecular weight polymers, petroleum oils tend to shear back very quickly. In other words, they lose

their ability to maintain their viscosity at high temperatures.

 

Synthetic basestocks, on the other hand, are much less prone to viscosity loss at high temperatures. Therefore, lower molecular weight polymers may be used as VI improvers.

 

These polymers are less prone to shearing, so they are effective for a much longer period of time than the VI improvers used in petroleum oils. In other words, synthetic oils do not quickly lose their ability to maintain viscosity at high temperatures as petroleum oils do.

 

Cheers

Simon

[kevin 0]

have no doubt that you know your stuff from reading these threads!!!!!!.

but could i ask "in your opinion" what the ideal oil would be for a g60???

thanks in advance

regards kevin

[oilman 0]

Stock road car a 5w-40

 

Modded and/or track car a 10w-50

 

Cheers

Simon

[Paulr1980 0]

Out of interest Simon, you mention that it would be better to go for a 10w over a 5w for a track based G60 but surely by the theory, it's better to run a 5w 50 on a track based G?

 

In a sense, running an oil that is less resistant at start up but has the ability to not thin out to much at higher temps. Why the need to go up to a 10w?

 

Just curious. Great reading though, thanks!

[oilman 0]

a 10w-50 will stay in grade far longer than a 5w-50 as the gap is less.

 

On track when the car is properly warmed a 10w or 15w is fine, the key is the step to sae 50 which can withstand higher temps than an sae 40.

 

The "w" number is cold crank and not relevent to the upper engine temps

 

Cheers

Simon

[Guest]

very interesting reading there, cheers :)

[Kevin Bacon 5]

They are in fact polymers that are heat sensitive and uncurl with heat. These are contained in all multigrade oils of any quality.

 

Except Polyol Ester basestocks, say Redline.

 

So what's the difference between all these producers of ester based oils? Redline, Motul, Silkolene Pro S, Royal Purple et al.

 

It's also worth mentioning that some oils evaporate more than others. This is the oil vapour that comes out of your breather hose and condenses onto colder metal and plastic parts around the intake system.

 

I would like to hear your opinions on Royal Purple oil actually as I've read some very bold claims about it. Is it marketing hype, or a genuinely ground breaking product?

 

Probably the best "all round good egg" oil I've used in any of my VW engines was Castrol Formula RS 10W/60. Can you still get it?

I see there is a BMW approved Castrol 10W/60 on your site, but only lists BMW as having manufacturer endorsement. I suppose the 60 element is to prevent excessive consumption due to the piston clearances and very high revs, so it should work well with forged pistons, turbos and air cooled motors, which all have loose tolerances.

[oilman 0]

All oil companies make bold claims although some more than others.

 

Through chemical basestock testing we have found the Silkolene Pro S, Motul 300V and Redline to be the best under lab tests for quality and addatives.

 

All of these may be overkill for stock road cars but would not harm, quality never does!

 

Castrol RS 10w-60 has been replaced with Castrol Edge Sport 10w-60 (NOT Edge 10w-60) on our website, this is the same oil.

 

Regards

Simon

[Kevin Bacon 5]

Cheers for clarifying that Simon.

 

Edge sport isn't badly priced actually, might give it a whirl!

[oilman 0]

Why not, we have 10% off the stuff at the moment too!

 

Cheers

 

Guy.

[madscientist 0]

whats the difference between oils that are labled for diesel and ones for petrol?

 

I should know about this stuff really as i used to do a bit of Kinimatic viscosity and do Ricaard (spelling) anti-ware testing and use a shell 4 ball tester as well.

[Kevin Bacon 5]

Yeah I've always wondered that too, especially as I've seen VW and stealth use Quantum Silver in both petrol and diesel engines.

 

As a guess, I reckon diesel oil has increased amounts of anti-sludging additives in it due to the amount of blow-by soot diesel engines produce?

[oilman 0]

Most oils these days are for both despite what they say on the tin.

 

The way to check is to look for the ACEA spec on the back of the tin, ACEA A# is for petrol and ACEA B# is for diesel and most will show both these specs.

 

There are some specials out there, and these are for VAG and BMW diesels with particulate filters etc, bit again can be used in a petrol if need be.

 

The only real use for disel specific oils these days is for commercial use, trucks, generators etc and they tend to have stonger detergents etc in them.

 

Cheers

 

Guy.