When you notice that your oil is a bit lower than it ought to be, you can add more to make sure your engine is well-lubricated. Just be sure to check the level again in a day or two to make sure it has not dropped significantly, as this may indicate a leak.
When you get to the store to buy a quart or two, there will probably be an entire shelf dedicated to motor oil. It might be a bit overwhelming to try and determine which type you'll need for your car. The owner's manual will be able to point you in the right direction for your specific vehicle, but what do all those numbers actually mean?
Every bottle of motor oil provides you with two crucial pieces of information to determine if the lubricant is suitable for your vehicle - an API service rating and the viscosity grade. This information can be found on the "Service Symbol Donut," a round label that will have the API rating on top, the viscosity rating in the middle and the bottom will let you know whether or not the particular product is designed to conserve energy.
The API rating, classified by the American Petroleum Institute, lets you know whether the oil is appropriate for your vehicle based on when it was manufactured. It also indicates the quality of the oil. For example, oil for most cars and light trucks build between 2004 and 2011 will have the two-letter code "SM." Vehicles manufactured starting in 2011 will likely call for oil with the "SN" API rating. Some API ratings indicate use for older vehicles, and it is important that drivers get the right one as lubricants intended for old cars can damage newer models. For example "SE" oil, which is not for use in any car built after 1979, can damage newer vehicles.
In the middle of the "Donut" is the viscosity rating. Viscosity is the liquid's thickness - water, for instance, is far less viscous than molasses. The most common grades used in automobiles today are 5W-30 and 10W-30. The first number indicates the cold viscosity rating of the oil and the value following the "W" represents how the oil will act in higher temperatures. In cold-weather climates, lower cold viscosity ratings will help the engine turn over on frigid mornings.