The Professional Expert

Decoding the Oil Bottle

The numbers and letters on the oil bottle refer to how thick the oil is, the higher the number, the thicker oil.  For example “SAE 30” is referred to as “30 weight” and is thinner than “SAE 50” or 50 weight oil. Where SAE stands for Society of Automotive Engineers.

Most cars today use “multi grade” oil.  These have numbers like “5W30” or “20W50.”  The oil behaves like the first number when it is cold and like the second number when it is hot.  Pretty nifty, huh?

When you look in the owner’s manual under the “Service Specifications” section you will find the recommended oil for your vehicle.  They may recommend different weight oils based on the temperature outside, and therefore they may want you to use a different oil in the winter than in the summer.

This is the table from our owner’s manual. As you can see, we use 5W30 during all seasons. In the text it tells us that we can use 10W40 in an emergency, but we should replace it with the 5W30 as soon as we can.

Above is the circle seal you will find on a bottle of oil. In the middle, you find the weight of the oil. In the outer circle you see “API Service SM/CF.” And below the circle, you see “API Service SM, SL, SJ, CF.”

API stands for American Petroleum Institute. The other letters refer to the properties and additives that are in the oil.  The specifications with an “S” are for gasoline engines, and the “C” refers to specifications for diesel engines. 

As they come out with new specifications, they move the second letter to one later in the alphabet. For example, SJ is an older standard than SM. Your owner’s manual will tell you what specifications your oil needs to meet. 

You can use oils with a more recent specification. For example, if the manual calls for “SJ or better” then you can use any oil with a specification later than SJ such as SL or SM.

Tomorrow we’ll talk about different brands and synthetic oil…

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