What the numbers and letters mean
Engine oil used to be a lot simpler. The local auto parts store typically carried a few varieties of a few major brands, and that was about it. As engines have become more complex, engine oil has diversified to keep up with changing needs — as evidenced from all the specs printed on bottles of engine oil. Fortunately, there are industry standards that explain it all.
On the front of a typical engine oil container, you’ll see the oil’s specified viscosity rating. For our basic introduction to engine oil, just think of viscosity unscientifically as “thickness.” A few common viscosities include 0W-20, 5W-30, 10W-40, and 20W-50, though there are many more. These are multigrade oils, containing additives to tailor their viscosities to various engines’ requirements and ambient operating temperatures (although, on the latter point, the numbers themselves do not translate directly to thermometer readings).
Take the example of 5W-30 oil. The first number, 5, indicates the lowest operational cold temperature range, while the second, 30, signifies the highest operational temperature range. You may see single-grade oils with simpler viscosity numbers, like 20W, for instance. These have a limited operating range and only perform well for specific applications and conditions. Most modern vehicles are happiest with multigrade oils.
Whatever the engine oil’s viscosity, the numbers are usually preceded by “SAE,” which stands for the Society of Automotive Engineers, the organization whose viscosity standards determine the numbers. Incidentally, the prominent “W” is somewhat redundant in that it stands for “winter,” reconfirming the first number’s representation as the coldest operating range.
Some other letters you might notice on the container could be in the API “donut.” API is the American Petroleum Institute, which established voluntary certification standards in a partnership with major American and Japanese manufacturers. The letter code on the donut represents the oil’s performance level. For gas engine oil, you may see “API Service SM” as an example. A diesel-specific engine oil letter code might be “CI-4” or something similar. Generally speaking, oil with sequentially higher designation codes supersede previous codes and are compatible with engines built to run with oil from earlier designations.