By the early 1900s, there were dozens of car makers in the United States and even more worldwide. Several of these suppliers and auto parts firms have formed trade groups that serve their needs to market their business and increase public awareness about this emerging mode of transport. With this development came the need for patent protection. Common technological design issues brought about the need for engineering standards, and fortunately many automotive engineers have allowed an ‘open exchange of information' in order to broaden their individual professional knowledge base. With this The Society of Automotive Engineers was formed.
And like many car parts and automotive mechanisms, oils used by cars specifically both engine (motor oil grades) and transmission oils also became a subject of standardization. For this purpose, The Society of Automotive Engineers (SAE) developed a scale to rate them.
It is a given fact that motor oil thins as it heats just as it thickens as it cools. So it is important to be able to determine its viscosity — how with the right additives it could resist from thinning out too much in high temperature, or the rate it thickens as it cools. Knowing the right oil for you according to the condition you’ll be driving in (high or low temperatures) should be able to help use oil with the right viscosity to continue to efficiently lubricate your engine’s moving parts, not only for fuel economy but also to do away with premature wear and tear of you engine components.
One specific rating for motor oil is its viscosity ratings notated with the common “XW-XX.” The “X” preceding the W rates the oil's flow at 0 degrees Fahrenheit (-17.8 degrees Celsius) and the “W” is for winter. The lower the number for “W,” the less it thickens in the cold, meaning a 5W-30 viscosity engine oil thickens less in the cold than a 10W-30 and subsequently more than a 0W-30. So if you are in a colder place you would benefit from 0W or 5W viscosity oil.
A car in a hotter climate would then also need a higher number to keep the oil from thinning out too much. The second number, coming after the “W” rates the oil's viscosity measured at 212 degrees Fahrenheit (100 degrees Celsius). This indicates the oil's thinning resistance at high temperatures. So a 10W-30 oil thins out at higher temperatures faster than a 10W-40 will.
Some antique and vintage engines however use what we call monograde oils, which fall into two main categories. The ratings with a ‘w’ after them such as SAE 5w, 10w, 15w and 20w are exclusively suitable for use in winter time because they are generally thin oils good for use during winter or for cold starting.
The ‘summer time”’ grades on the other hand like SAE 30, 40 and 50 are more for higher temperatures or for use in an already hot engine. The limitation with single grade (or mono grade) oils is that they can only operate efficiently over small temperature ranges, which means a SAE 10w oil is fairly thin and good for cold starting but too thin for effective lubrication when temperature rises.
In one way this is the reason why monograde oils are no longer used in modern automotive engines. They may be only suitable for some vintage and antique engines. Straight SAE 30 oil is also often specified for small air-cooled engines that includes garden tractors, lawnmowers, gas-powered chain saws and portable generators. These SAE ratings/labels can be found on every container of reputable motor oils to guide you.
API Engine Oil Classification
Also one important thing to note, is also to go for an oil with the recommended weight from a brand that displays the starburst symbol that assures you the oil has been tested by the American Petroleum Institute (API). You can find an API donut displayed on the right that tells you if that specific oil qualifies for the current SL service rating (C for diesel engines), in addition to the viscosity number provided by the SAE (Society of Automotive Engineers). SL refers to a group of laboratory and engine tests, that includes the latest series for control of high-temperature deposits. The label on oil containers will also be able to tell you if the oil has passed the Energy Conserving test. Although it's not really a guarantee of better fuel economy, most of the leading motor oil brands have some viscosities that are so labeled.
As a trade association for the oil and natural gas industry in the US, API reunites different players in the production, refinement and many other activities around petroleum, like measurement methods in setting up industry standards for the energy conservation of motor oils. The two categories are set for gasoline and for diesel.
Category S for “Spark Ignition,” meant for Gasoline (for use of cars, vans, and light trucks with gasoline-engines) and Category C for “Compression Ignition,’ meant for Diesel (for use of heavy duty vehicles, like trucks, buses, etc.). For both categories the letter S or letter C is followed by another letter like SN or CG. The classification progresses alphabetically as the performance level of the lubricant increases, and each classification outdone those before it.
Like for instance if we have an oil labeled SM, it can also be used for vehicles which request API SL and SJ. But still you have to pay attention to the construction year of the engine because modern chemistry might not be adapted or may no longer be compatible with old engine designs and materials used to make that engine. Like for example, specifications from SA to SH (meant for 1995 and older engines) are already obsolete and that means that you cannot get any official license from the API. SL are for 2004 and older gasoline-engines, SM for 2010 and older gasoline-engines and SN are for modern gasoline-engines. These are the currently recognized API classes and previous classifications are obsolete.
It’s also important to note that oils meeting the latest API classification like API SN, may be used in any engine and that includes older engines where earlier category oils were recommended, unless the engine manufacturer specifies a “non detergent” oil. Non-detergent oils like SA and SB are not recommended for use unless specified.
Resource Conserving API SN Service Category oils include the performance properties of each earlier category and are designed to offer improved high temperature deposit protection for pistons and turbochargers. It could also provide improved oxidation resistance and fuel economy, more stringent sludge control, emission control system and seal compatibility, and lastly, protection of engines operating on ethanol-containing fuels up to E85.
But for “performance” engines or fully race built engines the engine protection requirements are above and beyond API/ILSAC requirements so there are available specialty oils out in the market with higher than API that allow phosphorus levels (examples are Racing Oils and AMSOIL’s Z-Rod oils). Phosphorus is a key anti-wear component for motor oil and is usually found in the form of zinc dithiophosphate (ZDDP). But due to chemical poisoning that phosphorus has on catalytic converters all the current gasoline categories have placed limitations on phosphorus components for certain SAE viscosity grades.
As mentioned earlier, oils in the “C” (Commercial/Compression Ignition) category are designed for diesel-engine commercial service like heavy-duty trucks and other vehicles with diesel-engines. They are namely CA, CB, CC, CD, CD-II, CE, CF, CF-2, CF-4, CG-4, CH-4, CI-4, CI-4 PLUS and CJ-4. When it comes to this category not all supersede one another. But just the same, these classification systems for diesel oil aim to help motorists choose the right oil for their driving condition. The oil option depends on the outdoor temperature, the engine and the type of driving the engine must withstand.
Classifications CH-4, CI-4, and CJ-4 are currently recognized by API and they created a separated CI-4 PLUS designation in conjunction with CJ-4 and CI-4 for oils that are able to meet certain extra requirements. This label can be found in the lower portion of the API Service Symbol “Donut”.
CH-4 oils that were introduced in 1998 were designed for high-speed, four-stroke engines to meet 1998 exhaust emission standards. They are specifically compounded for use with diesel fuels ranging in sulfur content of up to 0.5% weight. CH-4 can be used in place of CD, CE, CF-4, and CG-4 oils. CI-4 oils, on the other hand, was introduced in 2002 and are for high-speed, four-stroke engines to meet 2004 exhaust emission standards implemented in 2002. They are especially formulated to maintain engine durability where exhaust gas recirculation (EGR) is used and are intended for use with diesel ranging in sulfur content of up to 0.5% weight. CI-4 can be used in replacement of CD, CE, CF-4, CG-4, and CH-4 oils. Some CI-4 oils may also pass for the CI-4 PLUS designation.
CJ-4 oils are made for 2010 model year engines on-highway and for Tier 4 non-road exhaust emission standards, in addition to previous model year diesel engines. These oils, on the other hand, are designed for use with diesel fuels ranging in sulfur content of up to 500 ppm (0.05% by weight). But using these oils with greater than 15 ppm (0.0015% by weight) sulfur fuel may impact drain interval and/or exhaust after-treatment system durability.
CJ-4 oils are also able to effectively sustain emission control system durability where particulate filters and other advanced after-treatment systems are being used, among other properties like being able to provide optimum protection against piston deposits and viscosity loss due to shear. API CJ-4 oils exceed the performance criteria of other oil categories and can effectively lubricate engines calling for the other API C category oils. But when using CJ-4 oil with higher than 15 ppm sulfur fuel, you must consult the engine manufacturer for service interval.
Lastly, there are also oils that meet “Multiple performance requirements“ levels, where oils formulated for diesel engine service are also able to meet gasoline engine service. For these types of oil the designation is “C” category first followed by the “S” category.
When you are choosing a transmission or gear oil you might be wondering what GL-1 and GL-4 stands for and what the differences are between the two. Choosing the right classification is a must since they both have different properties and erroneously using one for the other could lead to serious damage to your vehicle. API classification subdivides all transmission oils into 7 classification: GL-1, GL-2, GL-3, GL-4, GL-5, GL-6 and MT-1.
GL-1 oils are designed for vehicles for light conditions so they are composed of base oils generally without friction modifier additives but may contain small amounts of antioxidant additives, depressants, corrosion inhibitors and anti-foam additives, while GL-2 oils are formulated for moderate conditions. GL-2 already contains anti wear (AW) additives and are designed for worm gears and for proper lubrication of agricultural machine transmissions. GL-3 contains up to 2.7% anti-wear additives and are designed for moderate conditions in lubricating bevel and other gears of truck transmissions.
GL-4 oils are formulated for various conditions – light to heavy and contains up to 4.0% effective anti-scuffing additives. They are often used and recommended for non-synchro gearboxes of tractors, trucks and buses and for main and other gears of all vehicles.
GL-5, on the other hand are oils for severe conditions. They compose up to 6.5% effective anti-scuffing additives. The higher the Extreme Pressure (EP), the higher the GL category becomes. The major difference between GL-4 and GL-5 gear oils is the amount of EP additives (Sulphur/Phosphorus containing products) being used. The additive prevents the occurrence of micro-welds on the gear flanks at the local high temperatures which prevail in EP circumstances (temperatures well in excess of 800℃!)
With that GL-5 has about twice the amount of EP additives compared to GL-4, which is why it is often used in high-pressure circumstances such as in a front axle and rear axle differential. Sulphur/Phosphorus additives however can react aggressively towards bronze and copper so it can be disastrous for synchromesh rings of a gearbox. So unless the manufacturer recommends it, a GL-4 is the best choice over GL-5.
Also GL-4 is best for hypoid gear service when they are under severe service but are without shock loading, while GL-5 is more suitable for hypoid gear service also under severe service and shock loads and not for use in a gearbox.
GL-6 oils are designed for very heavy conditions and contain up to 10% high performance anti-scuffing additives but it is important to note that it is not so much in use any more as it is considered that class API GL-5 is already enough to meet requirements for the most severe conditions.
Lastly, MT-1 oils are formulated for thermal stability, having 10% EP additives and friction modifiers for non-synchromesh manual gearboxes in heavy duty service vehicles such as bus and truck fleets.
There are a wide variety of options available when it comes to picking the appropriate oil for your vehicle’s engine so choosing which one is best for you might seem like a challenging task. There is loads of information that can serve as your guide and it is always helpful to know a thing or two about the fluid you’re putting in your vehicle, but the first and the most basic step is important, and that is to look in your car’s manual. Your manual will be able to tell you the recommended oil weight for your vehicle whether that's a standard like 10W-30 or something more out of the ordinary and it will also be able to tell you which transmission fluid you should be using for your vehicle.