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API Motor oil service rating donuts

API Motor Oil Service Classifications

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The service rating of motor oils is classified by the American Petroleum Institute (API). The program certifies that an oil meets certain OEM quality and performance standards. The service rating is shown in the API "Service Symbol Donut" on the product label. There may also be an "API Certified for Gasoline Engines" seal on the label.

SN Rated Motor Oils

The latest service category rating for gasoline engines starting 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.

Aeration control, the reduction of tiny air bubbles, is a renewed concern because modern engines demand that oil serve as a hydraulic fluid in cam phaser devices, variable valve actuators, timing chain tensioners and hydraulic lash adjusters that allow for variable valve timing. These increased demands cause engine oils to be stressed more than ever before.

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

motor oil
When choosing a motor oil, make sure
it meets ALL performance requirements for your vehicle.

GM dexos Motor Oils

For 2011, General Motors announced a new oil requirement called "dexos." GM says their new oil performance specification is better than the new GF-5 specification, which also went into effect in 2011. GM says dexos is required in all 2011 and newer GM engines, and is backwards compatible with older engines that use SM oils.

There are two versions of dexos: dexos1 for gasoline engines and dexos2 for diesel engines. The specification calls for a high quality synthetic base stock with additives that provide high temperature, high sheer characteristics to reduce friction for better fuel economy, to reduce piston ring deposits and sludge, and to extend oil life (necessary for use with GM's Oil Life Reminder System).

Because it uses high quality synthetic base stocks, dexos and other brands of oil that meet GM's dexos specification are more expensive than conventional motor oils. GM is licensing oil brands that meet their specifications. Pennzoil Platinum and Quaker State Ultimate Durability both claim to meet the new dexos spec in their SAE 5W-30 viscosity grade motor oils.

SM Rated Motor Oils Now Obsolete

The previous API service category rating for gasoline engines was SM, introduced in November 2004 for 2005 and newer engines. SM-rated oils along with the previous SL (2001) and SJ (1997) ratings, are also backwards compatible and can be safely used in older engines with exceptions (see update below). But the opposite is not true. Older obsolete service classifications (SH, SG, SF, etc.) may not meet OEM lubrication requirements for newer engines. Likewise, API SL oils should not be used in 2005 and later vehicles, and SJ oils should not be used in 2001 and newer vehicles.

Motor oils that meet the now obsolete API SM rating may also meet the International Lubricant Standardization and Approval Committee (ILSAC) GF-4 specifications, which some European and Asian auto makers require.

One of the major changes with the current SN motor oils is that the amount of phosphorus anti-wear additive allowed in the oil has been reduced to 800 ppm (parts per million). For more information on this subject, see ZDDP - What is it & Why do you need it?


Diesel Motor Oils

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

A new diesel oil category called PC-11 will appear in 2016 or 2017. The new diesel oil category will have two subcategories: PC-11A (which may be renamed CK-4) for current and older diesel engines, and PC-11B for 2017 and newer engines only. The new PK-11 oils will be lower viscosity, fuel-saving oils with better high temperature lubrication and evaporation qualities.

Energy Conserving Motor Oils

API also gives oils an "Energy Conserving" rating if the oil meets certain criteria for reducing friction and oil consumption, and improving fuel economy.

Watch Out for Poor Quality Oil in Quick Lube Shops

In recent years, there has been a growing problem of quick lube shops selling motorists poor quality oil. The American Petroleum Institute (API) did a nationwide survey in 2013 to check the quality of oil being sold by quick lube facilities. Oil samples were taken from bulk dispensers in quick lube shops and analyzed in a laboratory to see if they meet API quality standards, OEM performance requirements and SAE viscosity grades. The survey found that 1 out of 5 samples FAILED one or more of these tests! In other words, a lot of quick lube shops are selling poor quality oil that does not meet minimum service specifications or is not the correct viscosity grade. For more information on this issue, see Motor Oil Matters.

New Motor Oils Are Bad for Older Engines with Flat Tappet Camshafts

camshaft wear
The lobes on this 289 Ford Mustang flat tappet cam suffered excessive wear because the motor oil did not contain adequate levels of ZDDP anti-scuff additive.

If you are driving an older classic muscle car or hot rod that has an engine with a flat tappet camshaft, you should be aware of the fact that today's SM and SN rated motor oils contain much lower levels of anti-scuff additive called "ZDDP" (Zinc Dialkyl Dithio Phosphate). The level of ZDDP in current motor oils has been reduced to no more than 0.08% phosphorus to extend the life of the catalytic converter. Phosphorus can contaminate the catalyst over time if the engine uses oil, causing an increase in tailpipe emissions.

The lower ZDDP content is not harmful to late model engines with roller lifters or followers because the loads are much lower on the camshaft lobes. But on pushrod engines with flat tappet cams, the level of ZDDP may be inadequate to prevent cam lobe and lifter wear. In some cases, cam failures have occurred in as little as a few thousand miles of driving! This is even more of a risk in engines if stiffer valve springs and/or higher lift rocker arms are used.

To avoid such problems, you should add a ZDDP additive to the crankcase, or use an oil that meets the previous SL service rating, or use diesel motor oil or racing oil that contains adequate levels of ZDDP to protect the camshaft and lifters.

If you are installing a new camshaft in the engine, be sure to use the cam manufacturers assembly lube and follow the recommended break-in procedure. But you will still need to add ZDDP to the crankcase or use an oil that contains adequate levels of ZDDP for continued protection.

New Motor Oil Classifications Coming in 2020

The American Petroleum Institute (API) and International Lubricant Standardization and Approval Committee (ILSAC) have developed oil performance standards for new motor oils that should be available in early 2020. Some of these next generation oils have lower multi-viscosity ratings (0W-16) to reduce friction for improved fuel economy. They also include special additive packages to prevent engine wear and to reduce the risk of Low Speed Preignition (LSPI), a problem that can occur with late model gasoline direct injection engines when the engine is pulling hard at low speed. The additives in these new oil classifications have been developed for high output turbocharged gasoline direct injection (GDI) engines.

The new API oil classification is "SN-PLUS" while the new ILSAC standards will be GF-6A and GF-6B.

GF-6A oils are for the newest gasoline direct injection (GDI) engines, and will also be backwards compatible with engines that currently use GF-5 oils.

GF-6B oils will NOT be backwards compatible and will include some of the new ultra-thin 0W-16 oils that are currently undergoing testing and certification. These oils will be for future engines not yet in production. These oils are expected to provide a significant increase in fuel economy over today's low viscosity 0W-20 and 5W-20 motor oils.

For more information on the new oil GF-6 motor oil classifications, see the following:

ISLAC GF-6 Motor Oil Specification Update February 2020.

Get Ready for GF-6 Motor Oils (Motor magazine ).

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ZDDP - What is it & Why do you need it?

Motor Oil Viscosity

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