American Petroleum Institute (API)

Besides the SAE viscosity of an oil, API ratings are the most commonly used and referred to in the US when deciding if an oil is right for a particular application. Although there are other motor oil specifications that are sometimes used to determine a motor oil's quality and it's appropriateness for a particular application, SAE viscosity and API rating are what most people look for on a bottle oil.

The most current (as of November 2007) API specification for gas engine passenger car motor oils is category "SM". Any API classification that begins with an "S" references an oil designed for gasoline engines. Diesel oils, on the other hand, will have an API specification that begins with a "C". For instance, the most current API spec is CJ-4.

Typically, each new specification that is released is more stringent than the last. Stricter high temperature requirements. Tighter volatility limits. Lower deposit buildup allowances. You get the idea. Of course, this may not ALWAYS be the case, but it is in most cases.

Generally, more recent specifications are designed to be "backwards compatible" with previous specifications. In other words, an SM rated oil can be used in a vehicle which originally called for an SL rated oil (notice that the second letter in the specification changes - letters found later in the alphabet represent a more recent specification). This is true of diesel oil API specs as well.

How Is An Oil Classified as API "XX"

Much testing must be done to determine whether an oil actually meets the qualifications established by the API classified as SM or SL or SJ, etc. For instance, in order for an oil to qualify as an API SM rated oil, it must be tested for high temperature oil thickening, deposit buildup and wear, low temperature sludge, varnish and wear development, high temperature volatility, foaming and rust prevention characteristics, change in low temperature viscosity with usage, etc.

If an oil is run against all of the required API SM test parameters and passes all tests, then the company can say that the oil is qualified to be used for applications calling for an API SM oil. If the oil manufacturer would like to put the actual API "certified" seal on their bottle, then they must pay a fee to the API for that certification as well as a "royalty" on each bottle sold.

This is a minimum specification that is used by the oil and auto industry to help consumers purchase oils that will be adequate for proper protection of their vehicles and equipment under most circumstances. This assures that consumers are not purchasing oils which will not hold up under the stress of continued usage.

However, if consumers routinely run their vehicles under higher stress than normal or would like to take advantage of longer oil drain intervals or enhanced engine protection, oils that simply "meet" these specifications will not offer the performance characteristics that are necessary.

It is at this point that consumers would be required to do their own research to find an oil that will "do more" than simply meet the minimal required API specifications. A study and comparison of the technical specifications of individual oils may be necessary to find an oil that will exceed the API specifications in one or more areas, depending upon the requirement of the application.

For instance, a vehicle owner may live in a very cold climate and may be looking for enhanced cold temperature performance, beyond what is measured by the API standards. In that case, investigating an oil's pour point, cold cranking and cold pumping characteristics would most likely be in order.

Alternatively, if a vehicle owner lives in a very hot climate and does alot of heavy hauling or racing, then cold temperature performance characteristics may be less important. High temperature performance might be more of an issue, in which case an investigation of an oil's flash point, fire point, volatility and shear stability would be in order (not to mention some other test results, if they can be conjured out of the oil manufacturer).

The point is, API ratings should be looked at as a minimum standard that assures adequate performance and protection under normal circumstances. Anyone that is looking for performance and protection beyond what is simply adequate should be looking for oils that can demonstrate that they far exceed what is required by the latest API specification.

Always Use the Most Recent API Spec - OR MAYBE NOT

I mentioned earlier in this FAQ document that more current API specifications are backward compatible with previous specifications (API SM oils can be used in vehicles calling for SL, and so on). However, it should be noted that, although an oil that meets a more recent API specification can generally be used in an application that originally called for a less recent specification, it is not ALWAYS the recommended option.

For instance, the new API CJ-4 specification for diesel engines was designed to address issues with the newer EGR (exhaust gas recirculation) systems found on the newer diesel engines. The newer EGR units place a HEAVY burden on the oil in a few critical areas, and the new specification had to account for that.

However, by modifying the oil characteristics to meet these new challenges, certain concessions had to be made. One such concession that is of particular concern is the significantly reduced allowable levels of sulfated ash in the oil.

This reduction was necessary in order to increase the lifespan of particulate traps associated with the new emissions systems. Unfortunately, it comes with a trade-off. Sulfated ash levels are directly proportional to the level of detergent additive in an oil. By lowering the sulfated ash limits, this new API specification effectively limits the amount of detergent additive that can be placed in an oil.

Since, in most cases, an INCREASE in detergent additive results in a longer lasting, cleaner running oil, a DECREASE in detergent additive will have the opposite effect.

This is potentially a considerable issue and may result in an oil with has shorter drain intervals and will not keep an engine as clean and rust/corrosion free as oils meeting the previous CI-4 specification did (TBN - total base number - of an oil also decreases as sulfated ash limits are lowered - meaning less ability to neutralize acids which cause corrosion inside an engine).

Similarly, API SM oils have much tighter limits on phosphorus levels - down to about 800ppm at this point, I believe. This was done with the intent of increasing catalytic converter life since some studies have shown that as oil consumption occurs, the phosphorus additive in the oil passes out the exhaust and poisons catalytic converters.

However, there is a problem with this approach. Although API SM oils CAN be used in vehicles calling for previous performance specs, it MAY not be the best approach. You see, phosphorus is one crucial piece of a two piece anti-wear puzzle. ZDDP is the acronym for zinc dialkyldithiophosphate, an additive that is included in motor oil formulations to increase the anti-wear properties of the oil.

ZDDP actually forms a "plating" over the metal surfaces within the engine so that, if metal to metal contact ever occurs, it is the plating that receives the brunt of the wear, not the actual engine components. By reducing the amount of allowed phosphorus in motor oils, the new API specifications are effectively minimizing the levels of anti-wear additive that can be used, thereby reducing the overal anti-wear properties of the oil.

In years past, phosphorus levels in some oils have been as high as 1300-1600ppm or more (although many oils were NOT that high). A drop from well over 1000ppm to 800ppm or less is a significant reduction in anti-wear additives that could result in more wear in your engine. So, the decision to use an API SM rated oil is one that should be taken very seriously.

But What About My Catalytic Converter?

If you're concerned over using an oil that may contain higher phosphorus levels than API SM allows, I'm not going to tell you not to be concerned - only that you should do your homework. There are premium oils on the market that have YEARS of solid data showing that even with higher levels of phosphorus, their oils do not "poison" converters.

The amount of phosphorus that ends up passing through the exhaust is not only related to the amount of phosphorus additives, but also to the volatility of the oil and the additives IN the oil. Most oils on the market have NOACK volatility levels between 12 and 15% (15% is the maximum volatility currently allowed by the API limits for gas engine oils). Many synthetics out there might be in the 9 or 10% range, which is a considerable improvement and could result in an equally significant decrease in phosphorus passing through the catalytic converter.

Some synthetics have NOACK volatility levels as low as 5 or 6%, which is a HUGE decrease over the 12-15% seen in most oils. Thus, even with higher phosphorus levels, the amount of phosphorus actually passing into the catalytic converter is still considerably less.

So, What Does That All Mean?

In the end, just understand that you can always select an API certified oil and receive adequate protection, as long as you change oil frequently enough. However, IF you are looking for something more than just an average oil, it will take a little more in-depth research and analysis on your part to find what you're looking for. Fortunately, "The Motor Oil Evaluator" offers a tremendous tool to get you started in the right direction.