Engine Oil Viscosity
 

Engine Oil Viscosity

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 oilman@opieoils.co.uk

With thanks to John Rowland of Fuchs/Silkolene

Cheers

Tim and the Opie Oils team

Brilliant!

Glad you like it

I use 5w40 in my standard blob sti.... What are your thoughts?

too thin, I wouldnt use it in mine. Local subaru specialist near me claims the only time he see's customers coming in foe a rebuild is when they have been using a 5w.

That's exactly what you should be using:thumb:

I always thought 5/40 was the same viscosity as 10/40 when warm just thinner to start with? Therefore more protection from cold??

Very good explanation Tim. :thumb::notworthy

I think there have been some misconceptions that multi grade oils such as 10w-40, 15w-50 actually become thinner during winter and thicker at 100 deg cel.

"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."

I basically alternate between these 2 oils:

Motul 300V 10W-40 and 300v 15W-50.

I mainly use the 40 for everyday use. but switch to the 50 when i track the car or will be using it harder than usual.

Now with the car getting on in age (over 60k miles now), the consumption rate for the 40 is approximately half to 3/4 quarts every 4k miles. Which i think is totally acceptable?

Should i really be using the 50 from now on? There is almost no consumption with the 50. The only reason i don't like using the 50 is because the car does feel less responsive - which is to be expected.

Should i go for 40 or would my engine thank me for using 50 even for daily use?

It's for a choco pistoned EJ257 08 STI by the way. :lol1:

Forgot to mention that the average temperatures here is 25-32 deg cel year round

How can that be right when 5w30 is the subaru recommended oil?

dont know mate, was told although it says that in the manual, its not actually the recommended oil, its the lowest you can get away with. Apparantley the 5w is too thin for the barings and causes them to fail prematurely.

I checked with a few different tuners (some very respected) and they all said the same thing.

I use 10w60, but I do drive my car very hard.

Mine always run on 5w40 shell helix, 68000 no smoke and uses very little between changes.
But it does have quite and easy life, just the odd blast :)

I was also told to run 10w50 in my daily driver 2.5 sti, by a subaru engine builder/tuner so thats what i run

2.5,s are inevitible to fail no matter what oil you run.... But i do know a guy that runs 10/50 on a 440bhp blob sti on standard internals with no problems

I use Gulf Competition 5w 40 in my standard Spec D STi 07 plate.

Matt at Opie suggested that to me, and it seems fine.

I'm about to change mine to 15-50 and i'm just coming up to 100k was using 10-40 but did a bit of reading and it seem higher milage cars are better run on 15w mine is garaged at night and the temps here just about make it to zero overnight for a couple of months.

No smoke and uses less than 1/4 litre every 1000/1500 miles dependant on use, lot's of short 10/15 mile journeys so I think that has something to do with it.

generally speaking we dont get the - temps in the UK to warrant switching oils winter to summer

a std car used lightly is a totally different proposition to the same car put through arduous track type use - IMO getting the right oil for your use is far more important than worrying about extra "cold protection"

+1 gulf comp 5/40 recommended by tim oilman my01 wrx 300bhp 66k running sweet :))

oilman isnt a subaru expert though, hes salesman and ive seen some of these posts repeated on other car forums. I'll choose to use oil thicker than a 5w as recommended to me by people that know these cars inside out.

Not all Subaru experts recommend thicker oil, just to put that one straight.

I havent really said anything wrong, phoned rcm, they recommended a 15w50 I think, been speaking to andyf recently in regards to getting my car mapped, he recommended nothing less than a 10w, local specialist 15w50 again.

Im not saying you have, just that not all experts have problems with 5w oils.

My 380bhp STi hawkeye runs on millers 10w 40 with 55k miles on the clock, have never noticed that it uses/burns any oil. Due a oil change in the next few weeks and going to try 10w 50!!!

Been told by a number of Subaru owners and Subaru garages not to run 5w oil as its to thin , was also talking to a someone the otherday who had a customer come in with bad engine slap on a 2.5 which was using 5w oil and advised them to change to a 10w oil and the slap went away!!!!

So if I use the oil Subaru recommends my car will explode?:confused:

Yes and you will probably die!!!!:cry:

So basically using a thicker oil will not only save your engine it will also save your life...

They should put that on the bottle, I'd buy it.

+1. I'd much rather take advice from Alyn than someone who's a mere salesman.

There is much more to an oil than the viscosity.

HTHS and viscosity index to name a few.

A very good site is http://www.bobistheoilguy.com

For an everyday car I also use 5w-40 in winter (Silkolene Pro S now or Shell Helix Ultra in the past) and 10w-50 in the summer (Silkolene Pro S).

E.g. check the kinematic viscosity of different oils at 100C is:

Shell Helix Ultra 5w-40: 13.1 cSt
Silkolene Pro S 5w-40: 14.89 cSt
Silkolene Pro S 10w-50: 18.17 cSt

at 40C

Shell Helix Ultra 5w-40: 74.4 cSt
Silkolene Pro S 5w-40: 92.4 cSt
Silkolene Pro S 10w-50: 116.5 cSt

The lower the kinematic viscosity the more fluid the oil is.
Ideally you need an oil with a low kinematic viscosity at lower temperatures and higher at high temperatures.

That's just one of the parameters that characterise an oil though and I am learning myself but for my needs I think a 5w-40 protects an everyday car better than a thick 15w-50 for example.

When you go on track that's a different matter.

Get the correct oil for what you use your car mostly for. There is no such thing one is best.

Maybe oilman will say the same thing again in 10yrs time

10/40 is the same viscosity as 5/40 when warm 5 just being thinner on start up