Your vehicle's most important fluid is the engine oil. It is a necessity for your engine to fully function properly. You might know how to change your oil but do you understanding what the oil rating system is that is shown on your oil container's exterior? This article will clarify the rating system of oil.
Oil is defined as a liquid lubricant used to reduce friction between moving parts. This reduction of friction is what allows your engine to work correctly. As oil breaks down it is not able to do its job as well and that is why oil changes are needed.
With that basic knowledge regarding oil we will move on to the industry set that provides the rating systems. The groups that standardized the oil rating system are the Society of Automotive Engineers (SAE), the American Society for Testing and Materials (ASTM) and the American Petroleum Institute (API). These three entities collaborated to provide the rating system as it is today.
Now that you have some simple information on oil itself and the industry groups that created the ratings system let's take a look at the actual rating system used for oil. There are two types of ratings to be familiar with when discussing engine oil. These types are service (or quality) and viscosity.
In the 1940s the service ratings were classified as Regular, Premium or Heavy Duty. In the 1950s these classifications were replaced with ML, MM and MS the order was to show the increasing quality of the oil. Finally in the 1980s there was another revision of the service ratings that continues to be used in present day.
The current service ratings came about by testing done by the American Petroleum Institute (API). The service ratings classification system starts with SA and proceeds to SM currently. According to the API's motor oil guide SM, SL & SJ are the current rating levels that are being used for gasoline powered vehicles. Ratings below SJ are considered obsolete which means they are for older vehicles. API recommends motorists refer to their owner's manual before deciding which service rating is correct for their vehicle.
The second oil rating system is the designated as viscosity. The viscosity rating will be the number printed on the motor oil container that starts out SAE. SAE is the group that classifies the viscosity type after testing the oil at different temperatures.
There are different grades of viscosity that have been defined. Six of these grades end in the letter W, with the W standing for winter. These six levels start with 0W and continue up in steps of 5 ending with 25W. The remaining five grades start at 20 and proceed to 60 in steps of 10. The higher the number the higher the viscosity level is the general rule.
The viscosity level measures the oil's thickness and ability to flow at certain temperatures. The purpose of having a viscosity rating for engine oil is to assure that whatever temperature the vehicle's engine is operating at that the oil will be able to flow smoothly through the whole lubrication system. At high temperatures oil tends to thin out while at low temperatures oil can get too thick. That is why there are two types of viscosity ratings: single and multi-viscosity.
Single viscosity oil varies in thickness for wide variety of temperatures or climates. You can spot single viscosity oil by it only having one number, such as SAE 40. This single number is due to that fact that oil is a simple, straight grade of petroleum.
Multi-viscosity oil slowly changes in viscosity as the temperature changes. Multi-viscosity oil has been designated to have two numbers such as SAE 15W-40. The double number refers to the oil behaving in two ways. It will act like a conventional 15W oil would at 0 degrees Fahrenheit and like a conventional 40W oil would at 200 degrees Fahrenheit. The advantage to consumers using multi-viscosity oil is that it can sustain ample thickness at high engine temperatures while it withstands the tendency to thicken at very low temperatures.
The oil rating system is made up of two parts the service rating and the viscosity grade. Both ratings systems are continually being tested and revised as new information and technology come to light. The best way to insure you are buying the proper engine oil for your car is to consult the owner's manual that came with your vehicle.
As a side note the API has a different guide for diesel powered engines. This classification system goes from CA to CJ-4. CJ-4 was introduced in 2006 for high speed 4-stroke engines.
A good tip on remembering which rating system is for which type of oil is that S = spark ignition while C = compression ignition. The API also gives oils an "Energy Conserving" rating if the oil meets certain criteria for reducing friction and oil consumption, and improving fuel economy.
It you need help determining the right oil for your vehicle and cannot find your owner's manual than speak to any reputable mechanic for help. A mechanic should even be able to explain the basics of the oil system as we have here and tell you which engine oil is right for your vehicle and why.