Demystifying Engine Oil – What Those Numbers Really Mean
In a car engine, oil has two important purposes. First, it provides lubrication and prevents friction between the moving parts. Second, it helps absorb the heat produced by the engine and moves it away from the engine. As a general rule, when you’re purchasing car oil, it’s best to use the oil recommended by your car’s manufacturer.
The SAE – or Society of Automotive Engineers – has established a classification system for motor oils. The lower the grade number of the oil, the thinner, or less viscous the oil is. Conversely, the higher the number, the thicker and more viscous the oil is. The grade number may range from 0 to 60 in increments of 5 or 10.
However, car engines don’t run at a constant temperature. As the engine heats, plain or single grade motor oil becomes thinner – think of butter melting and becoming thinner when placed in a hot pan. It’s important that the oil not be too thick to work when the engine is cold, nor too thin to work when the engine is hot.
To address this issue, the SAE tests single grade oils at winter temperatures. This is where the “W” in oil grades comes from – W means winter, not weight. The same oils are tested at high temperatures, up to 212 degrees Fahrenheit. Oil that has the grade of 10W should function as 10 weight oil at either end of the temperatures at which it is tested. The grade number may range from 0W to 25W in increments of 10. Single grade motor oils don’t handle great temperature changes or extremes well, so it’s used primarily in vintage or classic cars, and some types of equipment, such as lawn mowers.
When a special additive is introduced into plain or single grade motor oil, the resulting product is known as multi-grade oil. This additive is called a viscosity index improver and is designed to allow the motor oil to better handle changes in temperature. This is where we get the second number in the grade of multi-grade oil. The second number refers to how the oil behaves when it is hot – for example, 10W-40 oil behaves as 10 weight oil when the car is cold and 40 grade oil when the engine is up to its usual operating temperatures.
Of course, the grading system discussed above only refers to petroleum products – there are also several synthetic products that you can consider. Two of the advantages of using a synthetic product are that it functions better at extremely cold temperatures that can cause traditional motor oils to solidify, and that it doesn’t break down as quickly when exposed to usual engine operating temperatures.
Whatever type of engine oil you choose, it’s important to change your engine oil regularly and frequently. In fact, manufacturers recommend changing the oil every 3,000 to 5,000 miles for petroleum based oils or from 10,000 to 15,000 miles if you’re using a synthetic product. Check your oil regularly, and if you see any signs of oil break down – including darkening and increased viscosity – have your oil changed. It’s almost impossible to change your oil too frequently or too aggressively, and remember that any oil change should also include the installation of a new oil filter.