Porsche Enthusiasts Club Forum

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

Interesting read, and thanks for sharing. Be good for some folks if you would add a next chapter, covering the likes of SL specification and how the zinc content of oils can impact the life of engine components, detergent mixes and also stuff like mineral vs. synthetic, high mileage engines, laying-up oil and so on. There is a wealth of complexity with oil... and I suppose that's why it's big business for some.

Good to know why picking up a 5l container from Wilkos might be just dandy if you change your oil every 3 months or 2000 miles, vs. spending quite a lot of money on some fancy pants branded synthetic... and why running high-tech very thin oil might sound great in the shop and then bugger your engine in the real-world

Hi

Glad you like the article, but the other bits you mention could turn into several articles.

ZDDP is something that we hear about all the time and I swear some people would try to use liquid ZDDP in their engine if they could. Yes it helps, but there are other things in the oil that are just if not more important. If you use a basic oil, then a high level of ZDDP is necessary with certain engines, but with a better ester based oil, the ester content will help as much (or more than) the ZDDP. Unfortunately, a lot of the information going around about ZDDP is incorrect/out of date and it's not as important as many people think it is.

Here is something from John Rowland of Fuchs about ZDDP. It was written mainly for the A Series engine, but the general points about ZDDP are relevant.

1)       It is true that reduced ZDDP (zinc di-alkyl di-thio phosphate) levels have been proposed to improve exhaust catalyst life, because the phosphorus compounds in burnt oil do tend to de-activate the active sites on the catalyst matrix.
2)       Even in 1960 when the A-Series was 8 years old, good quality oil (API SB) would contain some ZDDP of a primitive sort, equivalent to 300-500 parts per million of zinc. Current API SG/SH/SJ oils contain much superior types of ZDDP (it’s all to do with the length and branching of the alkyl side chains) at levels equivalent to 1000-1200ppm zinc. The point is, even the reduced ZDDP oils will still contain 600-800ppm zinc, so they’ll be much better than any 60’s or even 70’s oil.
3)       ZDDP is not the only anti-wear compound in modern oils. The sulphur in the calcium or magnesium sulphonate-based detergents (present at high levels in modern oils) also has some anti-wear effect, and some oils contain phosphorus-free anti-wear agents such as molybdenum dithio-carbamates. (Nothing to do with MoS2. Don’t even think about it.) Some expensive oils also contain load-carrying synthetic ester lubricants.
4)       The type of polymer used to generate the ‘multigrade’ performance of an oil is also important. There are expensive ‘shear-stable’ types that resist thinning during use, and cheap ones that do not! The way to really reduce wear is to maintain ‘hydrodynamic’ or thick-film lubrication at the tappet/cam contact. This depends on viscosity and rubbing velocity, and is highly desirable from a wear point of view because there in no metal/metal contact. In ‘boundary’ or thin-film lubrication there is contact, and this is where the surface-active anti-wear compounds such as ZDDP and molyDTCs do their stuff. Obviously, a fairly high viscosity shear-stable multigrade is an advantage. There is a test called the ‘High Temperature High Shear Viscosity’ (HTHSV) which is run at 150C in a close-clearance taper bearing rig. It is perfectly possible for a shear-stable 15W/50 for example to be twice the viscosity of a run of the mill 15W/50 in high temperature and high velocity/close clearance situations. A good 20W/50 or 15W/50 will have a HTHSV of 4 to 6 Centi-Poise (cP or mPa.s) units; a poor one could be as low as 2.8. Incidentally, even expensive ‘wide-range’ multigrades such as 0W/40 and 10W/60 tend to have average HTHSVs because they contain a lot of polymer, and more polymer means more sensitivity to shear effects.
5)       It is all very well for me to say ‘buy a shear-stable oil’ but in some parts of the world, the USA in particular, this is not easy. Standard fill for American cars is 10W/30 or 5W/30 fuel-economy grade which is about as shear stable as Swarfega. This is fine for engines that spend 99% of their life at 5 to 15% of their rated output. An average car only uses about 20 to 25 BHP to move at 55MPH on a level road; maybe 30 to 40BHP if it has idle 4-wheel drive and the aerodynamics of an outside toilet. (Facts which are not popular with macho posers!). For the hard-working small classic engine which needs a UK-type better quality oil, a good choice is a motorcycle grade. Motorcycle engines are small, powerful, high-revving, and have a small oil capacity. Also, most of them have combined engine/gear lubrication, and there’s nothing worse than gear teeth for shearing down a poor quality multigrade. Look for 20W/50, 15W/50 or 10W/50 grades which pass the Japanese ‘JASO MA’ specification. This includes a shear stability test. JASO MA was brought in by the four main Japanese bike makers as a response to problems with low-viscosity ‘fuel economy’ oils, mainly in the USA.
6)       Our top man in the USA tells me that there is the ‘oil shop ritual’ in the USA, were well-meaning owners take their cars for an oil change about every 3000 miles. The oil used in these places is usually poor quality, reflected in a price of around $8 pre US gallon. I think the justification is that cheap oil every 3000 miles is better than oil costing 4 times as much every 9,000 miles. It isn’t.
7)       When 2 dissimilar metals (even if they’re only slightly dissimilar) are in contact, and some moisture is present, an electrical cell is created. So, given time, a cam in contact with a tappet will, if there is some moisture in the oil film, initiate a region of corrosion. The lessons are obvious: Always run the engine hard enough and long enough to get the oil hot. This drives off water and traces of fuel. Use your car regularly to circulate the oil and renew lubricant films. 3000 miles per year is far less damaging than 300 miles per year. Never start the engine, run it for a couple of minutes, then switch off and leave the poor thing to stand idle for weeks!
       And finally….do not, EVER, ever even think about using snake oil, magic additives, bolt-on charms or whatever that claim to rebuild the engine from the inside, reduce friction to less than zero, and so forth. Over the past 30 years I’ve analysed on average two a year of these useless gadgets and concoctions, and without exception they do nothing….. if you’re lucky, and if you’re not they damage your engine. If they demonstrate anything at all, it is the ignorance of their purveyors and the gullibility of their purchasers. Needless to say, the Internet is awash with them.

The bit at points 4 and 6 is relevant to the debate over using a good oil once or changing a cheap oil several times, and I'd agree with John. Basic oils do not protect as well, even if changed on a regular basis

This will help explain the benefits of synthetics over mineral oils, and also helps to explain why more expensive oils can be worth the money.

http://www.opieoils.co.uk/pdfs/tech-art ... c-Oils.pdf

If an engine has been treated well, then there is no need for a different oil when it gets to a high mileage. If you're using a decent oil, it will contain agents that help the seals and should have prevented excess wear. If there has been a lot of wear over the miles, you might need a thicker oil to prevent/slow consumption. Using too thick an oil will lead to increased wear though, so you want to get it right.

Hope that helps a bit.

Cheers

Tim