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.
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.
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.
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?.
For diesel engines, API has a separate rating system. The current category is "CI-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.
API also gives oils an "Energy Conserving" rating if the oil meets certain criteria for reducing friction and oil consumption, and improving fuel economy.
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.
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.