What is the meaning of 15W40?
This post may seem like going back to basics but I am always surprised by the amount of people who don't know what is written on a bottle of oil.
If a bottle of oil doesn't have any basic information, then you should not buy it and look for something else.
Ignore the marketing blurb on the label, it is meaningless, and I will explain later what statements you should think about.
All oils are intended for an application.You wouldn't put an Automatic Transmission Oil in your engine.It's important to know what the purpose of the oil is.
Most oils on the shelves today are Multigrades, which means that the oil falls into two different grades.10w-40 etc.
It was 50 years ago that multigrades were first developed to avoid using thinner oil in winter and thicker in summer.
In a 10w-40 it simply means that the oil must have a certain maximum flow at low temperature.The lower the number, the better the performance.
The 40 in a 10w-40 means that the oil must fall within certain limits.All oils that end in 40 must achieve this fixed limit.A 30 oil is thinner than a 40 oil at 100 degrees.Your handbook will tell you if a 30, 40 or 50 is required.
The performance of the oil, whether they have met or passed the latest tests, or whether the formula is out of date are all important.All good oils should contain at least one of the two specifications that you should look for on the bottle.
The basic is that it is split into two catagories for passenger cars.Most oils carry both petrol and diesel specifications.
Although suitable for some older vehicles, they do not provide the same level of performance or protection as the more up to date SL and SM specifications.
The more up to date CH4 & CI4 specifications provide the same level of performance or protection for older vehicles, but they are not as good as the specifications prior to CH4.
The performance of the oil is more specific than in the US.A is petrol, B is Diesel and C is Catalyst compatible.
The majority of oils in the UK mention VW, MB or BMW, but do not think that you are buying a top oil because of this.
Some older specifications can be achieved with the cheapest of mineral oils.The newer specifications are always better than the older ones.
If you are looking for a quality high performance oil, you should ignore some of the older specifications listed here.
The most accurate guidance I can give is above, however there is one final piece of advice regarding the labelling.
If you are looking for a fully synthetic oil, be aware that it will only be a semi-synthetic oil.
The cheaper the oil, the less expensive the ingredients and the lower the performance levels.
The Viscosity Index improves.As the engine temperature increases, the oils viscosity will decrease.This thinning is reduced by adding Viscosity Index Improvers.They are used in the production of multi grade oils.
VI Improvers are heat sensitive long chain, highmolecular weight polymers that reduce the viscosity loss of the oil at high temperatures.They are coiled at low temperatures and uncoiling at high temperatures.The thicker the oil, the larger the molecule which increases internal resistance.They fight back against the loss of Viscosity in the oil.
VI Improvers are prone to "shearing" with use which reduces their ability to prevent the oil from losing its viscosity.Shear stress can cause long chain molecules to be converted to shorter, lower weight ones.The shorter, lower weight molecules have less resistance to flow.
Increasing oil consumption and engine wear can be caused by the shearing of the oil.
VI Improvers vary in quality.Synthetic oils are likely to incorporate better VI improvers if the finished article is more expensive.The less VI improver an oil has, the better it will stay in grade.
Which oils need more VI Improvers?There are two scenarios where large amounts of these are required.
In "wide viscosity span" multigrades.I mean that the difference between the lower "W" number and the higher number is large for example 5w-50 and 10w-60.
0w 30 or 5w-40 require less VI Improvers and are less prone to "shearing".
Mineral and hydrocracked synthetic oils require more VI Improvers than PAO/Ester synthetic oil because they have a higher inherent VI to begin with.
It is a fact that many synthetics require less VI Improver to work as a multi grade and are less prone to shearing.
The oil is subjected to high temperature 150C and sheared at 1 x 106 per second.For normal bearing wear, a value of 2.8 is considered the minumum.The higher the number of shear stable the oil, the better it is for base stock quality.Silkolene and Motul.
Synthetics are good for 9000-12000 miles on the road.
The change periods will be determined by the quality of the oil.How stable is it?A decent race oil will take about 10 hours of hard track use and a semi-syntehtic or hydrocracked base stock will last about an hour before the oil begins to lose Viscosity.
The measure of a fluid's resistance to flow is called centistokes.The time required for a standard quantity of fluid at a certain temperature is calculated.The higher the value, the more fluid it is.
The value is meaningless unless accompanied by the temperature at which it is measured.In the case of oils, the Viscosity is measured in centistokes at 40 and 100 degrees.
All oils that end in the same number are around 18cst at 100degC.
In cold weather, can the oil flow quick enough to protect vital engine parts at start up, even though there is plenty of viscosity at 0degC.Not really!
If an sae 40 is 14cst at 100degC, how can a 0w oil be too thin?
The clever part is that thin base oils can be used so that start up viscosity is reduced to around 800cst and this obviously gives much better flow than a mono grade sae 40.
It's all about temperature, yes a thin base oil is still thicker when cold than at 100degC but the clever thing is that the chemists are able to build these oils out of synthetics again.
What are the parameters for the recommendations?Good cold start protection is something we always talk about, by this we mean flow so a 5w will flow better than a 10w and so on.The thickest you can use is 5w or 10w.It's important to protect the engine from wear.
We discuss oil temps, mod and what the car is used for.This is related to the second number xw-(XX) as there may be issues with oil temperatures causing the oil to be too thin and therefore the possibility of metal to metal contact.
If your oil temp does not exceed 120degC at any time, then a sae 40 is perfectly capable of giving protection.