What Does 'C' Stand For On Engine Oil Packaging?

There are a lot of vital fluids needed for your vehicle to run properly, and engine oil is right at the top of the list. This substance is crucial for keeping your engine and other components working properly, reducing friction and heat, as well as the damage that can come from an overabundance of both. When looking over the offerings from the best motor oil brands on the market today, it's clear that not all oil is the same. Certain vehicles and engines require specific oil types, which are noted on the packaging. For example, some engine oil features a prominent C on the packaging, but what does this mean for what it can be used in? 

This letter designation comes from the American Petroleum Institute, or the API in its shortened form, which has been around since 1919. In the over a century since its creation, it has standards within the oil and gas industry. That includes categorizing engine oil based on its appropriate use cases. For those in the C category, it's explained by the API that this oil is specifically for use within commercial diesel engines. Considering the differences between the machinery they're used in, it's no surprise that there are some major differences between diesel and conventional oil.

To make things even more complicated, not all C-grade oil is the same. There's an API-set hierarchy for this specific oil type that influences which scenarios it should be used in.

C-grade oil is a subcategory of motor oil designed with specific vehicles and components in mind, but even C oil isn't one-size-fits-all. The API outlines several different C oils to choose from, which vary in terms of a few different factors. Emission levels, diesel fuel sulfur content, the model year of the vehicle, and more influence which type of oil you'll want to use, and it's an understatement to say that you'll want to get the correct one. Using the right oil is just one of several engine care elements to keep your motor running long-term.

At the time of publication, the API recommends four different C oil types for use in modern vehicles: CH-4, CI-4, CJ-4, and CK-4. CH-4 is good for high-speed, four-stroke engines designed to meet 1998 exhaust emission standards, while CI-4 is good for four-stroke engines meant for 2004 standards. CJ-4 fits the 2010 model year on-highway and Tier 4 non-road exhaust emission standards, as well as those for previous model year diesel engines. CK-4 covers the same criteria for 2017 model year standards. There are numerous obsolete C-grade oil types as well that have since been phased out with the advent of new diesel technology and emissions guidelines.

While this covers the long and short of C-grade oils as set by the API, the story doesn't end here. It's worth mentioning that per another longstanding body that oversees oil production and classification, C has another designation.

The C designation — standing for "catalyst compatible" — provided by the European Automobile Manufacturers' Association (ACEA) is used to describe oils that are comparatively stable and are low in terms of friction and viscosity. These oils can be used in a variety of situations, including vehicles with diesel particulate filters, three-way catalytic converters, and diesel and gas engines that require lower SAPS (sulfated ash, phosphorus, and sulfur) levels. Keeping with the trend, these oils aren't equal by any means. The big distinction between the ACEA's C oils boils down to their SAPS content. 

While these additives are generally included to reduce engine wear and tear and increase lubrication, too much can negatively impact elements like particulate filters and catalytic converters. So as not to damage some engines while benefitting others, six different C oil types are outlined by the ACEA. C1 and C4 are categorized as low-SAPS oils, while C2, C3, C5, and C6 are considered mid-SAPS oils. However, just because they fit into these broader categories based on their similar formulas doesn't mean these oils are interchangeable. You should consult your vehicle's manual to ensure you're using the right kind of oil for your engine.

The minutiae of engine oil can be complicated, but it's undeniably vital. Understanding what letters like C mean is incredibly important to putting the right type of oil in your ride, as using the wrong oil in your vehicle can lead to a litany of problems.