Engine Oil Viscosity

[oilman]

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

[Thebusofwoe]

Nice. I learnt a bit about this when we went on the Castrol visit when i was an apprentice. But that's good to know that. Good info for all.

[Salazar]

Nice one Tim I understood the basics of oil viscosity, but great to read through a proper explanation.

[oilman]

Glad you like it

[handsomedanc]

Yeah good read.
Im currently running 10w 40 semi synthetic. i always assumed this was right for the car. im due an oilchange soon so with the cooler/bloody freezing weather not far away would i be better off going for something like 5w 40?

[oilman]

Hi

A 10w-40 is fine as a standard choice, but a 5w-40 would be an improvement as it will offer a better cold flow rate. 5w-40s tend to be synthetic as well, so will offer better all round protection.

Cheers

Tim

[handsomedanc]

Cheers for that Tim

[vtectom]

I was advised to use 10w/60 in my ex-race track car. Should I always allow the engine to warm up before driving it please?

[oilman]

Hi

A Prelude race car? I doubt it needs a 10w-60. A lot of tuners and specialists recommend a 10w-60 even though it can be harmful

http://www.opieoils.co.uk/pdfs/tech-art ... 60-oil.pdf

In many cases a 10w-50 or 5w-40/10w-40 will be preferable to a 10w-60, depending on oil temperature and the modifications made to the car.

Cheers

Tim

[vtectom]

I had a Prelude track car with a standard engine which I ran on 10w/40 with
no problems.

Now I have a Crx with an Ek9 engine running just over 200bhp.
It uses oil but doesn't smoke.
Spec is;
Full race port and polished cylinder head
ACL Bearings
ARP head studs
Alloy underdrive Crank Pulley
AEM EMS-4 Plug harness and AEM UEGO Gage Controller
2 ½” custom stainless steel straight exit exhaust
Four branch heat wrapped manifold
Hondata-P28 OBD1 to OBD2 ECU fitted with S300 Management System
Supertech-Block Guard
Supertech-flat bottom three way cut valves
Supertech -valve springs
Supertech-Valve spring retainers
Baffled sump.
Cooling:
Mishimoto Dual core aluminium Radiator
Hell- Oil cooler kit 19 row , braided lines
10” Lightweight electric fan.