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Selecting the Correct Engine Oil

Many of us purchase oil based on the recommendations of the vehicle manufacturer but do not really understand what we are purchasing, or why. To most of us, the different certifications and ratings on oil bottles look like a foreign language. However, if you do not understand these certifications and ratings, you may be using the wrong oil.

To start narrowing the vast selection of oils available, you must first understand how they start out. Every brand of oil starts out with a base lubricant. The American Petroleum Institute (API) labels these base lubricants as groups I, II, III, IV, and V. Groups I, II, and III are mineral-based lubricants. Group III is the most refined and the highest quality of the mineral-based lubricants. Groups IV and V are full synthetic base lubricants and are of higher quality than groups I, II, and III.

Unfortunately, oil companies are not required to disclose which base group their product starts with, but it has been my experience that the brands that use high quality base lubricants will disclose the information on their bottles. To be on the safe side, avoid purchasing brands that do not disclose the API’s group rating on their labels.

A knowledge of an oil’s viscosity rating (measurement of the stickiness and consistency of a fluid) will also help you select the proper oil for your application. The Society of Automotive Engineers (SAE) uses a rating system to display an oil’s viscosity in a “numberW - number” format (e.g. 5W-30) or SAE number (e.g. SAE 60). The first number in a rating like 10W-30 is the winter performance rating. In the simplest terms, the lower the number before the W, the better the oil will perform during a winter start-up. The number after the dash rates the oils performance at normal engine operating temperatures. In simplest terms, the higher the second number is the better it will perform in a hot running engine. The rating displayed in SAE 60 format (also called straight-weight) carries no winter rating. If you live in a climate that is warm all year, you would not need an oil with a winter rating. However, an oil rated at 10W30 will perform the same as an oil rated as SAE 30 in a warm climate.

The American Petroleum Institute assigns service classification numbers to engine oils. This classification means the oil meets or exceeds Original Equipment Manufacturer (OEM) quality and performance standards. To date, there are 12 different classifications assigned by the API. Prior to 1930, the API created the first service classification (SA) and every time the API released a new classification, the second letter changed to the next letter in the alphabet –excluding the letters I and K. Here is a complete list of the API classifications and the years they were in service:

  • SA – Obsoleted in 1930*

  • SB – Obsoleted in 1951*

  • SC – Obsoleted in 1967*

  • SD – Obsoleted in 1971*

  • SE – Obsolete in 1979*

  • SF – Obsoleted in 1988*

  • SG – Obsoleted in 1993*

  • SH – Obsoleted in 1997*

  • SJ – Currently in use but rated for 2001 and older

  • SL – Currently in use but rated for 2004 and older

  • SM – Currently in use but rated for 2010 and older

  • SN – Introduced in 2010 and still used

*Oils considered obsolete by the API are still sold today, be sure to read the labels to avoid purchasing an inferior oil that may damage your engine. To be on the safe side, avoid purchasing or using an oil if you cannot find any of these classifications on the label.

The Japanese Automotive Standards Organization (JASO) developed a system to rate an oil’s ability to resist wet-clutch slippage. The JASO ratings can be broken down into two basic groups, JASO MA and JASO MB. MB rated oils contain friction modifiers and will work well in gasoline engines but can make a wet clutch slip under load. MA rated oils do not contain friction modifiers that can make a wet clutch slip. If you are unsure, ask a service technician, or your vehicle manufacturer, to find out if your machine has a wet clutch.

In summary, always refer to your owner's manual for the recommended oil weight and oil change intervals. When selecting an engine oil, always look for the “API Doughnut," along with the other ratings covered in this article. Now that you have a better understanding of what the certifications and ratings on an oil bottle mean, you can purchase your next oil change with confidence.

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