Why are Oils Numbered
Stores rarely have enough varieties of oil to choose from! Shelves can stretch across walls, lined to the edges with colorful jugs. Inspect them closer, and you will peek into the world of grades, rates, and quality checks. It is a new language and a harsh realization that you simply cannot dunk the first jug into your car.
Before frustration overwhelms you, let us explain.
Oils are numbered to make it easier for you to pick the particular grade of oil compatible with your car. Depending on the grading organization or the standard being followed, the grades will tell you different information which goes into judging whether the oil is suitable for your vehicle or not.
The most frequently-considered and crucial measure is the oil ‘weight’ or viscosity.
What is Oil Weight
Oil weight is merely the jargon for the oil’s viscosity. Viscosity is a measure of an oil’s resistance to flow, primarily due to internal friction. It is related to how thick or thin the oil is. More viscous oils are ‘heavier’ and take longer to flow (like honey). On the other hand, less viscous oils are thinner and flow across easily (like lemonade).
Keeping track of oil viscosity is important because it determines when and where you can use the oil. For example, if you live in a place where the winter temperature can drop to 0⁰C, you will need an engine oil that can flow at that temperature.
However, external temperature is just one of the many determinants that guide the viscosity of engine oil. Inside an operating engine, several factors can come together to influence the viscosity. The oil that you buy will need to stand up to the engine’s forces.
Factors Affecting Oil Viscosity
The viscosity of oil can change due to situational or environmental changes. Some of these factors are determined by the manufacturing process. Other factors are influenced by your car, geographical location, and even your engine’s state. Others still are the result of changes that occur due to time.
Some of the critical factors that affect viscosity are:
- Temperature
Viscosity and temperature are inversely proportional. If the temperature rises, viscosity is reduced. Similarly, viscosity increases (i.e., a lesser rate of flow) if the temperature goes down. Engines experience huge temperature swings every time the car is used. An example would be when a car is started. In response to the immediate rise in temperature, engine oil tends to become less dense and flows easily.
- Pressure
Viscosity is also inversely proportional to pressure. If pressure increases, viscosity will decrease, and the fluid will flow faster. In the context of your car, engines use pressure to move oil and lubricate its surfaces. A viscous oil will naturally come under this pressure and flow more easily.
- Base Oil Mix
The stock oils mixed to make the base oil determine the initial viscosity of the oil. Depending on the level of refinement, individual base oils have different viscosities. The process of mixing creates a product with optimum viscosity, lubrication, and stability.
- Additives
Oil additives work with temperature to create a significant change in viscosity. Additives that play a significant role in stabilizing oil’s viscosity are the Viscosity Index Improvers and the Viscosity Modifying additive. For example, oil
- Age of Oil
Oil does not stay the same forever. Over time, oil collects engine impurities, residues, and dirt, changing its viscosity. Less refined base oils are the most likely to break and collect inside the engine. Viscosity additives too become weaker over time. They react with the oxygen in the engine’s heated air and form oxidized compounds that collect on the surfaces of the engine as undesirable sludge. This process accelerated with age, and oil will need to be changed to prevent damage to the engine.
As we can see, a wide range of factors can impact the fluidity of engine oil. As car owners, we have to choose from racks of oil jugs for oil that lubricates engines well and is also compatible with the engine.
SAE and Standardization
The Society of Automotive Engineers, or SAE, was founded in 1905. It started as a trade group of engineers from across the world who wanted to solve mutual design and technical problems that riled the automotive industry. One of the significant contributions of SAE to the automotive world is their oil viscosity standard J300 which is used worldwide. It is an alphanumeric system to grade oil according to its viscosity at a range of temperatures and engine conditions. The grading, in turn, lets consumers know what a specific jug of oil is for. For example, someone living in a place with low winter temperatures can buy the oil graded to flow at the low temperatures they may encounter. SAE grades are based on industry-standard tests, which contribute to the value of the grade.
The following section discusses the components of the SAE J300 system and the tests used to ascertain oil’s grade.
What is ‘W’ and How Oil Viscosity is Measured
Since temperature can drastically change the oil viscosity, the grading system has two types of measurements- one for summer and the other for winters. Under the SAE J300 system, ‘W’ is used to indicate engine oil fluidity at low winter temperatures. The numeric value before W is the minimum temperature the oil will stay fluid at.
SAE’s standard SJ300 lists four tests that determine the viscosity of the oil. Two of the tests are used to calculate the winter viscosity, while the remaining two calculate the viscosity at high engine temperatures.
Low-Temperature Cranking Viscosity
The test subjects the oil samples to a high shearing rate at temperatures from -10C to -35⁰C. The test looks at how well the engine can start at low temperatures.
- Low-Temperature Pumping Viscosity
In cold weather conditions, oil can cool to form waxy particles. These come together to form a gel that restricts the flow of oil into the engine. Especially in the early stages of the engine starting, before the oil has had time to warm up, such a phenomenon causes oil starvation in the engine. The test studies the rate of wax particle formation in engines at low temperatures. In the test, oil is slowly cooled from an initial temperature of -15⁰C to -40 ⁰C.
- Kinematic Viscosity
It is a high-temperature test measuring the rate of flow of oil under gravity. However, this test is not particularly important since the only time the engine encounters such a situation is when it is dripping down back to the sump. While the oil is heated, the situation is hardly critical.
- High-Temperature High Shear Rate Viscosity tests
The test is significant and far more telling of the oil’s capacity to perform well under realistic stressful situations found in engines. The test is conducted at 150 ⁰C, which is the average engine temperature. It comprises several tests to emulate the kind of shear the oil would encounter in the pistons, bearings, and valves.
The final grading is done in two sets. The winter grading goes from 0W to 25W in increments of 5. The grading for higher temperatures is from 8 to 60. This is done in two parts. The first is from 8 to 20 in increments of 4, while the remaining range (20-60) is in increments of 10.
In practical terms, lower numbers indicate less viscosity for winter oils, allowing the oil to flow without freezing up. For higher temperatures, a higher number shows the oil’s viscosity at the engine’s running temperatures.
Grading System Explained
Under the SAE system, there are two categories of engine oil.
- Single grade
They are mostly mineral oils without viscosity additives. The lack of additives limits their use to a particular seasonal range. They are also called ‘straight weight oils’ and need to be changed twice a year. They have a single number indicating the viscosity.
Measuring system
Single values show the viscosity. If the oil is for hot weather conditions, it has a single value without W. On the other hand, winter oil grades are followed by W.
Advantages
- Does not shear under stress from the engine
Disadvantages
- It needs to be changed depending on the season
- Multi-grade Oil
They are made with viscosity additives to make them functional across a wider range of weather conditions. Specifically, at low temperatures, multi-grade oils have the good flowability of thinner oils, whereas, at higher temperatures, they form a powerful lubricating film like heavier engine oils. They are called ‘all-season oils’ since they can adapt to seasonal changes and remain usable throughout the year.
Measuring system
They are written as two measurements. The first number ends with W and denotes the lowest temperature at which oil remains fluid. The second temperature is the measure of the fluidity of oil at temperatures encountered by the oil inside the engine. The
Advantages
- Better high-temperature performance
- Used in all seasons
- Oil warms up faster with minimum idling.
- Drains battery-less when the engine is started
Disadvantages
- If the engine is worn out, oil leaks may occur at low temperatures.
- Viscosity Polymers may shear under the stress and load of the engine, creating sludge.
Generally, thinner oils are preferred for better performance. But for engine oil, extremes of either are dangerous. If the oil is too heavy or dense, it will take longer to heat up and flow. The delay would compromise the functioning of the engine and cause delays or engine failures. If the oil is too thin, it would not provide adequate lubrication.
Now that we understand the viscosity grading system used for engine oils let us look at a typical label found on oil containers.
How to Read an Oil Label
If you have strolled past oil racks before, you may have noticed this label printed on the oil jugs indiscriminately.
- SAE Oil Grade
It denotes the grade of the oil according to the Internationally accepted SAE system.
API Service Classification Chart
Image Courtesy: Petroleum Quality Institute of America
- API and Service Classification
American Petroleum Institute (API) is another standardization organization like the SAE. The API is responsible for the service classification of oils. The classification shows what kind of engine the oil is compatible with. It consists of two letters. The first letter refers to the fuel used in the engine. S is used for Gasoline engines, whereas C is used for diesel engines. The second letter communicates what era of engines the oil is compatible with. Oil is classified from A to N here. A refers to vehicles from the 1920s, while N is for the latest vehicles. To know what kind of oil to use, check your car’s owners’ manual. This rating is essential for those dealing with vintage cars since the wrong kind of oil can cause damage to the engine. It is important to note here that the API rating has no bearing on the oil price.
- ACEA (European Automobile Manufacturers Association) codes
These are stringent performance grades. Oils can be put into categories depending on performance. Commonly used categories are A3/A5, A3/B5 for low viscosity oils, category C indicating the ash content in the oil, and category E for heavy-duty diesel engines.
Conclusion: The Key
Nowadays, there is an array of options to choose from and your preferences to consider. What will you use your car for, from road trips to wind down to office travel? Where do you live, from a road-blocked metropolitan to a winding hill town? What kind of prices and quality ratings would suit you?
The questions are endless, and the smallest ones can affect your oil choices. However, there is a way to make the task easier.
Consult your vehicle’s owner’s manual, a book that guides you on all you need to know about your car. It will advise you on setting up your car, cleaning, solving everyday problems, and regular maintenance and servicing. The manual will have clear instructions for what kind of oil will suit the car best- showing the compatible SAE oil grades and other rating and quality checks to look out for.