Engine Oil Viscosity

[oilman]

Viscosity is the most misunderstood aspect of oil and yet it is the most important.

Viscosity is the force required to shear (break) the oil at a certain speed and temperature. Oils work because they have viscosity; the drag of a rotating part pulls oil from a low-pressure area into a high pressure area and “floats” the surfaces apart. This is called “hydrodynamic lubrication” and crankbearings depend on it.

Oil must be capable of flowing at low temperatures, so that it gets around the engine in a fraction of a second at start-up and must protect engine components at high temperatures without evaporating or carbonising and maintain adequate (not excessive) oil pressure. Many people think that the thicker the oil, the better the protection, but if the oil is too thick, it will not flow properly, leading to reduced protection.

The numbers on every can of oil indicate its performance characteristics when new but there are many misconceptions on what these numbers actually mean.

For multigrade oils you will see two numbers (for monograde oils only one). The first is followed by a “w” and is commonly 0, 5, 10, 15 or 20. The second number is always higher than the first and is commonly 20, 30, 40, 50 or 60. The first and second numbers ARE NOT related.

The “w” number (0, 5, 10, 15 or 20)
When multigrade oils first appeared, a low temperature test called “w” (meaning “winter” not weight) was introduced. Using a “Cold Crank Simulator", the test measures the oils ability to flow at low temperatures. ALL oils are THICKER at low temperatures than at high temperatures but the lower the “w” number, the quicker the oil will flow at low temperatures.

The second number (20, 30, 40, 50 or 60)
This number is known as the SAE (Society of Automotive Engineers) number and is measured in “Centistokes” (cst) at 100C. 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 thicker the oil.

An oils cst at 100C determines its SAE rating within the following parameters.
SAE 20 = 5.6 to less than 9.3cst
SAE 30 = 9.3 to less than 12.5cst
SAE 40 = 12.5 to less than 16.3cst
SAE 50 = 16.3 to less than 21.9cst
SAE 60 = 21.9 to less than 26.0cst

ALL oils labelled 40 must fall within the SAE parameters at 100C so everything from a monograde 40 to multigrade 0w-40, 5w-40, 10w-40, 15w-40 or 20w-40 are approximately the same thickness at 100C.

Some oil companies label oils as SAE 35, 45 or 55, but as you can see from the above figures, there isn't a SAE 35, 45 or 55. This "could" be because they are approximately on the boundary of the two grades, but as we don't deal with any of those I can't really comment further.

Summary

Cold start.
A 5w-40 will flow better than a 10w-40.
A 10w-50 will flow better than a 15w-50
A 5w-40 is the same as a 5w-30

At operating temperatures.
A 10w-50 is thicker than a 10w-40.
A 15w-50 is thicker than a 5w-40
A 0w-40 is the same as a 10w-40

If you look above, you will see that the figures quoted do not indicate at all as to whether the oil is synthetic or mineral based... Well except for 0w oils as synthetic PAO basestock is required to acheive this viscosity.

Generally the oil you use should be based on the manufacturers recommendation found in the owners manual, but then modifications, climate and the type of use can affect that recommendation. If you are unsure of what is the correct recommendation for your car and would like to know more please contact us here [email protected]

With thanks to John Rowland of Fuchs/Silkolene

Cheers

Tim and the Opie Oils team

 

[clivehorridge]

Interesting read, I thought I knew what the numbers meant, I was correct on the first, but a million miles away on the second.

Ok, what is the consensus of opinion on additives. I remember in the old days, a big sales drive on products like Molyslip, STP and others. Now oil technology has advanced to where it is, are these additives recommended, to be avoided, or otherwise desirable in certain circumstances?

My interest is particularly focused on my old 340k km HZJ 80 engine, no turbo and as pretty basic as big old diesels can go. I don't abuse the engine with unnecessarily high revs nor do I tow heavy loads. Just some off-road over the mountains and relatively short trips during the week of 10 km per day on average. The car is used to commute every day throughout the year. Winter here can be -20C (and lower on occasions) and the summer is usually + 30C to 40C (and higher on occasions).

Cheers!

 

[Towpack]

This is how I've always read the oil numbers:

A 10w40 will have the viscosity of a 10 weight oil when cold but will thin no more than a 40 weight at full operating temp.
A 20w50 will have the viscosity of a 20 weight when cold but will thin no more than a 50 weight at full operating temp etc..etc.
Along with the API rating that's all you really need to know. The owner's manual will give you all the recommended numbers along with guidance for different climatic/enviromental conditions.
If you have an engine that runs 24/7 and never starts from fully cold then you could use a monograde. It's the cold starts that engines don't like where most of the wear takes place.

As far as additives go I have used several in the past but my opinion of them now is they're not really needed, especially if you buy a decent oil to start with and I don't necessarily mean the most expensive premium synthetics. JMO

 

[Himesed]

Really very good explanation.

 

[reyan]

Thoroughly i really like your post on Engine oil Viscosity.As we know that nowadays lots of Engine oil brands are available in market and some are give best performance.I want to tell that i used many brands engine oil for my car but Bosch Engine oil give best performance.

 

[Dave2000]

I only saw this thread by chance so thought I would add some recent findings to it.

A recent change of oil in my 80 has highlighted an issue. The issue is a 10/40 'synthetic blend' is thinner at higher temperatures than a mineral version of the same grade.

I had changed the oil and noticed I had around 5psi less at operating temperatures, I had used what I thought was my normal 10/40 mineral, I used a master oil pressure gauge and saw the pressure was down a little. But it was only after changing the oil filter in case it had a problem, then changing the adjustable oil pressure switch, and then the brand of oil that I came to the conclusion that synthetic blend 10/40 runs a little thinner than the mineral offering.

I actually changed the brand for another 10/40 synthetic blend oil, and the results were similar although the different brand gave around 2 psi higher reading. It seems there is some flexibility in the grading system?

As I cannot get either of the adjustable oil pressure switches to be reliable at 15 psi I am replacing with a fixed pressure version of a little over 5 psi, I am sure there is a fixed 10 or 15 psi switch out there somewhere with the same thread, but at the moment the buzzer is driving me bananas!

regards

Dave