Answers to Your Questions on Automotive Lubricants

Jeremy Wright, Noria Corporation

Over the course of the past few months, I have been instructing on-site machinery lubrication courses at what seems like a rate of one a week. Inevitably during a course, I will get multiple questions about automotive lubricants. I’ve decided to dedicate this article to answering some of the more popular questions that I get.

A Primer on the Auto Lubricating System

The first thing we need to understand is the mechanics of the automotive lubricating system. The system is designed to deliver clean oil to each individual component in the engine. It does this by first pulling the oil from the sump (oil pan) and into the pump.

The pump then forces the oil through the oil filter and pressure-feeds the main bearings and oil galleyways. From the main bearings, the oil passes through feedholes that have been drilled into the crankshaft and onto the bearings of the connecting rod.

The cylinder walls, piston pin and upper connecting rod bearings are all lubricated by the flinging of the oil as the crank rotates. The pressurized oil galleyways are responsible for the lubrication of the cam bearings, lifters, springs, timing chain and gears.

The lubricant has two primary roles to play. One is to control friction and wear. It does this by separating the two interacting machine surfaces with a film. Instead of having harsh metal-on-metal contact (known as a boundary condition), the lubricant provides a film that reduces the friction – in some cases, many thousands of times.

Another important role is temperature control. Heat is generated at the interaction point between the components. The lubricant is able to absorb this heat, carry it away from the component and dissipate it in the sump or in a cooler.

The lubricant used in today’s technology-advanced machines has to be very special. The American Petroleum Institute (API) has a program to certify that oil can meet the strict performance and quality standards put in place by the OEM. The service rating is shown in the API “Service Symbol Donut” on the product label. There also may be an “API Certified for Gasoline Engines” seal on the label.

Figure 1. API Oil Service Ratings

Stirring Through the Alphabet Soup

The newest service category rating for gasoline engines in 2011 model year cars and light trucks is “SN.” The API SN rating is equivalent to the new GF-5 oil rating by the International Lubricant Standardization and Approval Committee (ILSAC). SN engine oils are designated as Resource Conserving because they help improve fuel economy and protect vehicle emission system components.

These oils have demonstrated a fuel economy improvement in the Sequence VID test when compared with a baseline oil used in the Sequence VID test. Additionally, these oils have demonstrated in tests that they provide greater emission system and turbocharger protection and help protect engines when operating on ethanol-containing fuels up to E85.

Oils that meet the new SN and GF-5 motor oil ratings are designed to improve fuel economy, improve the life of emission components (such as the catalytic converter and oxygen sensors), and improve sludge, deposit and oxidation control. The oils also have better low-temperature viscosity, high- and low-temperature corrosion protection, better turbocharger protection, and improved filter-clogging protection.

The new SN and GF-5 rated motor oils are backward compatible, and may be used in 2010 and older engines.

For diesel engines, API has a separate rating system. The current category is “CJ-4” (introduced in 2007 for newer diesels that have exhaust gas recirculation). The previous CI-4 (2002), CH-4 (1998) CG-4 (1995) and CF-4 (1990) categories all can be used in older four-stroke diesel engines. CF-2 (1994) is the API classification for two-stroke diesels.

Here’s What to Look For

So, what should you be looking for when you go to buy engine oil? Let me point it out for you. The first thing is that you want to be sure and find the “API starburst”.


The API Certification Mark (the starburst symbol at right) tells consumers if an oil meets the most up-to-date requirements for passenger vehicles as outlined in the latest ILSAC specification. Oils that carry the API Certification Mark are energy-conserving and are suitable for all previous model years. The mark must be displayed on the front of licensed motor oil product packaging.

The next thing you want to consider is the service category. As previously mentioned, it is part of the API “donut” symbol (seen below).

The Service category (top of the donut) should be that which your owner’s manual states, or newer. The viscosity (center of the donut) should be consistent with the manual as well. The bottom half tells whether the oil has demonstrated energy-conserving properties in a standard test in comparison to reference oil.

If you can follow these simple steps and purposefully look for the proper markings, oil selection is quite easy. You will have to train your eye to look past the fancy marketing, colorful packaging and gimmicks. It’s what is written on the packaging that is of real importance.

Subscribe to Machinery Lubrication

About the Author