Lets clear up a common misconception about oil!

[oilman]

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

 

[E200 Simon]

Interesting reading Simon.

Slightly off the topic of Mercedes, I'm in the process of choosing oil for an M3 3.2 Evo engine for use in a Compact rally car. (Vanos issues etc.) I've seen your replies to this question on other forums, in relation to road cars.

I've had recommended the Castrol RS stuff, both in 5/40 and 10/60.

I'd appreciate your opinion.

Simon.

BMW Rallying.com

 

[oilman]

Hmm, well these two oils could not be further apart from a viscosity point of view.

sae 40 (0w-40) is 14cst at 100degC
sae 60 (10w-60) is 24cst at 100degC

That's 71% thicker at 100degC and the difference is larger at lower temps!

Now, unless the car has an M43, S54 or S62 (proir to 02/2000) you need to use the following viscosities 0w-30, 0w-40, 5w-30, 5w-40.

The exceptions listed above require 10w-60.

I would suggest BMW Approved oils are your best bet and the list is fairly extensive. You'll find technical data on a lot of them here:

http://www.opieoils.co.uk/lubricants.htm

Hope this helps

Simon

 

[E200 Simon]

Thanks Simon.

It has a 1999 S50.

Hmm, well these two oils could not be further apart from a viscosity point of view.

I realise this, which is why I'm confused about the advice I've received. There are contradictory recommendations from BMW too with this engine.

Would you expect my engine to require an oil that will operate at higher temperatures than the 'standard' oil? It won't be subjected to the sustained periods of high rpm in a way that a track car would, but it will be revved hard in bursts.

Tia.

 

[oilman]

It's down to maximum oil temps really and the most suitable oil should be selected based on the requirements viscosity wise of the engine concerned.

The question is, at what (sump) temperature is the oil at a viscosity that suits a modern high-RPM engine.

Present day designs seem happy on an oil viscosity of 10 to 15 cSt. (But many are OK on less than 10.) 30cSt is too high at high RPM. It can lead to foaming, air entrainment and cavitation.

............Temp. for 30cSt (Deg. C).......Temp. for 15cSt.......Temp. for 10cSt

5W/40..................71...........................90........................117.............
10W/40................70...........................99........................118..............
10W/50................80...........................109.......................130.............
10W/60................89...........................119.......................142.............

This shows that a 5W/40 or a 10W/40 is perfectly adequate for all engines except those that run an unusually high temperatures.

Also, a thick oil can lead to trouble unless properly warmed up before high RPM is used.

A shear stable proper synthetic 5w-40 will cope with oil temperatures of 120degC and without the power loss associated with thick oils.

Cheers
Simon

 

[sooper coop]

What would you reccomend for a well used 127k Petrol Six

cheers.

R.

 

[oilman]

Dpending on the condition of the car and how long you intend to keep it.

10w-40 semi-synthetic or
5w-40 fully synthetic

Cheers
Simon

 

[gpilko]

I've got a 99 SLK230 with 30k on the clock. When I recently had it serviced at an indy I asked what oil he recommended, fully or semi-synth. He said it's only really worth using the fully synth if that's what the cars been run on from day 1, otherwise just use the semi.

Does this make sense? Would I see any benefit from switching to fully synth over semi or should I just stick with the semi?

Thanks

Guy.

 

[oilman]

TBH no it doesn't.

It's always of benefit to run on fully unless the car is way past caring.

Synthetics protect better from engine wear, last longer and give better fuel consumption.

It's personal choice but they are superior in every way as they are not made of petroleum but in laboratories by chemists and for a specific application.

Cheers
Simon

 

[Frank Patten]

What about the rumour that putting synthetic into an engine that has had mineral in will cause oil leaks because synthetic is thinner at low temeratures?

I am just about to have my E220cdi serviced and I don't know what oil is in it at the moment as I have only just bought it. It has 62000 miles on the clock and is giving about 42mpg.

 

[Myros]

like all rumours

it has some basis in fact, but only some. If you are really worried, at your next oil change , do a flush, either with an additive, or with flushing oil. Do a normal oil change with your regular oil. At the following oil change, do another flush. That should have weaned your engine off all the accumulated dirt which might just be stopping your oil from leaking. So if you have no new leaks after a flush/change/flush routine, you are probably safe.
However, as with all advice, the caveat is,it will only leak if it is going to leak, and you will only find out if you make the change to synth.
I took the plunge and changed my 2 at 90k and 120 and no more leaks than there were before.
Try this link for more useful info

http://www.chris-longhurst.com/carbibles/engineoil_bible.html

 

[oilman]

What you need to understand is that most oils labelled synthetic are in fact modified or "hydrocracked" mineral oils anyway.

Basically a 10w-40 synthetic and 10w-40 semi-synthetic and 10w-40 mineral oil will all have the same viscosity at 0degC, 10degC, 40degC and 100degC.

The difference is purely the basestock used which is either petroleum or synthetic (pao/ester)

Compare these three at 40degC and 100degC and you'll see what I mean.

Motul 300V 10w-40 (pao/ester) synthetic
Silkolene XTR 10w-40 (hydrocracked) petroleum
Castrol GTX 10w-40 (mineral) petroleum

http://www.opieoils.co.uk/TechSpecs/Mobil and Motul/300V Chrono.pdf

http://www.opieoils.co.uk/TechSpecs/silkolene xtr 10w_40.pdf

http://www.opieoils.co.uk/TechSpecs/CastrolGTX10w-40.pdf

So basically it's complete nonsense!

Cheers
Simon

 

[tom7035]

What about the rumour that putting synthetic into an engine that has had mineral in will cause oil leaks because synthetic is thinner at low temeratures?

Changed from mineral to fully synth after a flush about ten months ago and had no problems Frank. Around 125K on the clock at the time. Regardless of mileage, I personally feel an oil with a superior performance just HAS to be better than a poorer one at ANY mileage.

 

[oilman]

Synthetics are fine even in older cars so long as the correct viscosities are used, it's just a superior lubricant at the end of the day.

Cheers
Simon

 

[Ray J]

Very interesting Simon.

So, it doesn't matter how old the engine is, switching from a mineral to semi' or full will give better protection.

One of my cars is a 1979 pushrod V8. I do less than 1000 miles a year in it and treated it with Slick 50, a few changes ago.

Is it worth the additional expense changing to semi or full?

Is it critical to flush before switching, as in switching from a mineral to a vegetable?

Ray

 

[mayolives]

Frank,
You ask, "What about the rumour that putting synthetic into an engine that has had mineral in will cause oil leaks because synthetic is thinner at low temeratures?"

I have been using 10-40 Castroil in my 560SEL for all it's life except for one oil change last summer when Moboil 1 synthetic was used by accident. This resulted in major oil leaks at both valve covers and the rear main engine seal.
The transmission had to be pulled to replace the rear main at a great expense. The leaks started within 500 miles after the oil change. It's Castriol 10-40 for me. all the way!

 

[tom7035]

Perhaps sheer coincidence. I can very well understand blaming the oilchange, with the timing of the leak/s occurring, but I really think if a leak was about to happen it would have happened anyway regardless of which oil was in the sump.

 

[Ray J]

Frank,
You ask, "What about the rumour that putting synthetic into an engine that has had mineral in will cause oil leaks because synthetic is thinner at low temeratures?"

I have been using 10-40 Castroil in my 560SEL for all it's life except for one oil change last summer when Moboil 1 synthetic was used by accident. This resulted in major oil leaks at both valve covers and the rear main engine seal.
The transmission had to be pulled to replace the rear main at a great expense. The leaks started within 500 miles after the oil change. It's Castriol 10-40 for me. all the way!

I've heard of this problem a few times.

Is it possible that a synthetic acts as some form of detergent. Cleaning too well? Thus allowing for the synth' to pass. Where a heavier mineral was unable to previously?

Ray 8)

 

[oilman]

There is no question that synthetics have better detergency and therefore can in some cases clean deposits built up over the years causing wider tolerances.

It's more about viscosity though and you need to understand that a 10w-40 fully synthetic has the same viscosity at ALL temps as a 10w-40 mineral oil so it's an old wives tale that an oil of the same viscosity will leak because the basestock is different!

Where I would not advise changing is in the case of say a car that is of an older design and uses for example 15w-50, don't change to 0w-40 synthetic as this is indeed thinner at all temps. In this case you should use a 15w-50 fully synthetic if you are going to change.

There are too many myths about synthetics and you need to bear in mind as mentioned above that viscosity is the most important factor.

Synthetics come in every viscosity from 0w-20 to 20w-60 so the correct one is out there.

Slick 50 is a waste of money and I would remove it and spend you're money on a decent synthetic oil.

Cheers
Simon

 

[haido1]

Simon,
Just a quick thanks for this great Post and the time you put into this.
Cheers
Tony