Chapter Three – Viscosity Modifiers and Engine Oil.
In chapter one we talked about how in the early days of the automobile, engine oils were very simple, basic refined fluids that helped keep moving parts easily sliding past each other. As time has passed, engines and engine oil has become more complex – allowing for different grades of oil with similar high temperature performance, but one being much close to the correct viscosity at startup. This is achieved using additive chemicals called viscosity modifiers.
Now I’m not going to go into a huge description of all the different types, shapes and styles of chemicals used to modify the viscosity, or into a long discussion about how they work. But what we can say is that oil makers use these molecules to change the way that the final oil blend responds to changes in temperature. This makes it possible to have 0W30, 5W30, 10W30 and so on.
Today’s engine oil is made up of several important components – the base oil mix, the viscosity modifying additives and other additives that have functions like preventing wear, cleaning an engine, making oil drain intervals longer etc. Right now, we will focus on base oils and viscosity modifiers.
To get the right viscosity at operating temperature (in our example this is a 10cSt oil at 212F or a XW30 oil) the oil maker will select a mix of base oils and a viscosity modifier (also sometimes called a Viscosity Index Improver –VII) to ensure the final blend hits that window. This may be a mixture of 6cSt and 8cSt base oil with a VII that puts the final blend into the XW30 grade. It could also be a mixture of 4cSt and 6cSt base oil with more VII that puts the final blend into the XW30 grade. The ratios of base oil and VII all impact the viscosity of the final blend. There are lot of reasons why an oil maker would choose a different mixture. Sometimes it is for better low temperature performance, sometimes it is due to cost or other manufacturing logistics. The trick is balancing the needed cold temperature performance while still hitting the XW30 grade target.
One of the downsides of using thinner base oils and VII’s to get to the high temperature performance window is that some VII’s wear out over time. This means that some oils with VII’s would thin out over time. This may be some of the reason for Porsche (according to some people) do not want to use a 0W30 but would rather a 10W30. If you are interested in what Porsche recommends look up the latest edition of the Porsche A-40 approved oils list.
The thinking here is that if the VII wears out then the oil might fall out of the XW30 window and instead become an XW20 oil which would be too thin based on the engine manufacturer’s required film strength and depth within the bearing clearances as manufactured. People who drive Porsche’s are often like to drive hard and fast. At high RPM’s, the internal oil friction in the journal and shearing in the bearings means the oil is even thinner, so there is a need to be especially careful with the “window” of acceptable viscosities. We will cover this in more detail when we talk about High-Temperature High Shear (HSTS) viscosity.
In most engines, having the VII wearing out is not really what happens in real life. While some VII’s do lose some of their effectiveness over time, in a normal engine, running under everyday conditions most are very robust and will keep your engine oil in grade. In fact, this is a requirement for a lot of engine oil specifications so the oil remains “in grade” over the life of an oil change interval.
There is also another class of viscosity modifiers called Pour Point Depressants (PPDs). PPDs function by getting in the way of forming wax crystals as the temperature gets cooler. Mineral based oils naturally start to form wax crystals and this can change how much the oil thickens when it starts to get cold. This is another way where the oil maker can influence the cold temperature performance (the XW part) without greatly changing the high temperature performance of the final oil formula.
Sometimes oil will thin out, but this is partly from fuel dilution and could also be partly by VII’s wearing out. What is more interesting is that with further use, the engine oil actually starts to thicken and this is much worse than the minimal thinning that would happen before. This thickening is usually caused as the oil oxidizes and can start to form varnish and sludge on engine parts. Changing your engine oil before it gets too oxidized can help keep engine parts clean and is one of the main factors that engine makers consider when recommending how often oil should be changed.
Another factor to creating varnish is oil volatility. Thickening of the oil (an increase in viscosity) can also result from the lighter fractions of the oil “evaporating” or “volatizing” at high temperatures. Most base oils are mixtures of both low viscosity and higher viscosity oils. It is the lower viscosity oils that have a tendency to evaporate and leave behind the higher viscosity oils with oxidation by-products, thus causing thickening of the engine oil over time. One of the things that an oil maker will do is balance the need for the lower viscosity oils with the risk of evaporation. The ideal oil is the one where this blend is closest to the right viscosity at the range of operating temperatures which balances things like wear protection, engine cleanliness, durability, and fuel economy.
Now viscosity is not only affected by temperature, it is also affected by load and forces called “shear forces”. Your engine oil may respond differently under different loading conditions. Shear forces are what happens when two surfaces move past one another. When those surfaces are separated by a liquid (like engine oil) it is said that the oil is “shearing” this does not mean the oil is breaking down or being cut up, it means that the viscosity is changing to allow more flow when the shear forces are high.
A good example here is to think of ketchup. When you turn the bottle upside down, the ketchup doesn’t like to move. In order for it to flow, you give it a shake and apply some shear forces to it. The ketchup then “shears down” to a level which allows it to flow out of the bottle. Your engine oil also works the same way, when high shear forces are applied your engine oil also thins out. This is a good thing at low temperatures, but at high temperatures it can be a concern. This is because you have two factors working to thin out the oil – temperature and shear. This is why there is a minimum HTHS requirement for every grade of engine oil.
Generally, engine manufacturers are now recommending lighter engine oils in both parts of the grade (i.e. choosing 0W20 over 5W30). One of the main reasons for this is because both oils are still going to be much thicker than the target operational viscosity at start-up, so if they can start out thinner, it will reach the operational viscosity window faster and consume less energy. This translates into better fuel economy. Engine oils will continue to be optimized lower and lower with engines designed to run on a 6 cSt oil instead of a 10 cSt oil at operating temperature in order to save fuel. While this is a good thing for everyone, these oils often require more complex formulas and better quality base oils.
This is where synthetic oils come in.Chapter Two