Engine Oil Viscosity

[oilman 0]

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 [email protected]

 

With thanks to John Rowland of Fuchs/Silkolene

 

Cheers

 

Tim and the Opie Oils team

[Kevin Bacon 5]

It's worth throwing flow rate and oil pressure into the mix as well.

 

Oil pressure is another misunderstood aspect of the engine's lubrication system. Oil pumps do not create pressure (neither do fuel or water pumps for that matter). They create flow rate. Oil pressure comes from the resistance to flow seen at the pump's outlet (like sticking your thumb on the end of garden hose). It's clearances between the metal parts and tiny oil galleries in an engine that create oil pressure.

It's precisely because of these tiny oil galleries in VVT units and other fine oil control systems in modern engines that OEMs specify a specific oil.

 

If an ageing engine suffers from low oil pressure, you can throw as many new pumps at it as you like, but the fact is it's worn out and needs rebuilding / replacing.

 

When an engine is new, sticking to the recommended oil is a good idea. But when it gets old and the clearances get bigger, stepping up from a 40 to a 50 can help restore some lost pressure and reduce oil consumption.

 

Flow rate can be an issue on old engines that have been run on mineral oil all their lives. The seals can harden due to the blow by acidity and if you stick a nice thin, fully synthetic in it with a different flow rate, it can weep past the seals.

Edited October 10, 2013 by Kevin Bacon

[wbx6dan 10]

Evening guys i have started a post recently ref the best oils to use,

I have read the posts in the sticky on here and multiple other forums from the "oil man"

who incidentally is very well respected everywhere!

 

Please put me out of my latest obcessional misery and answer me two questions for two different vehicles;

 

1, My corrado has a Nothelle G60 engine about 260 bhp with charge cooler and oil cooler plus cylinder head temperature gauge (this has a specially ground nothelle cam and the engine was referred several years ago and since been played with at jabba sport

It has used Quantum Synta gold 5/40 fully synthetic since from what i can see from the paper work i have etc

So silver or gold?

 

2, i have a oettinger wbx6 type 25 (6 cylinder 3.2ltr flat 6) with hydrallic tappets!

I am running castrol magnatex 10/40 semi happily good oil pressure doesnt use any either but i do have a couple of issues the first is the oil temperature got up to 120'+ the other day sitting for hours on the A303 ( i suppose it should be expected (and verified by other owners)fitting a 6 cylinder in to the hole where a 4 cylinder should be )however i feel this is partly due to the stage one on my radiator u/s! but the point is the specified oil should be castrol GTX2 which i believe was 15/40 and the main problem im having is after leaving the van for a couple of days or even moving it in and out of the garage the oil drains from the hydraulic tappets and i then have to do the drive of shame through my home town and for a few miles untill they self bleed at 3000rpm ! i spoke to a triumph stagg owner today and he recommended a 20/50 fully synthetic! from what i can see (Total Quartz seems to be a replacement for the castrol)

If the oil was thicker when hot would it stay in the tappets rather than draining out? or 10/50

If the oil were thinner would they bleed quicker? ie 5/40

 

I would very much appreciate an answer and will then sleep easier at night

thanks Dan

[wbx6dan 10]

bump? please