Car care FAQs

Engine oil breaks down over time. When it breaks down, it loses its effectiveness and can no longer properly protect your engine. In addition to lubricating an engine's moving parts, engine oil is designed to carry combustion by-products away from the pistons and cylinders. It is designed to deal with the small amounts of water that form as the engine heats and cools, and to collect the dirt and dust that enter the engine through the air-intake system. It also handles acids that are formed by the reaction between water and other contaminants. Sometimes there are even fuel leaks (fuel dilution) or coolant leaks that get into the oil system. 

As a car is driven, the level of contamination in the engine oil constantly increases. The oil filter removes particles as the oil passes through the filter, but over time, oil additives are used up and the oil itself can start to degrade (oxidize or thicken). At that point, the oil can no longer do its job and must be changed. The rate at which contamination and additive depletion occurs depends on many variables. Driving conditions may vary greatly and have a direct effect on the useful life of the oil. Other factors include the precision of ignition, fuel injection or carburetion adjustments, air cleaner service and the general mechanical condition of the engine. 

Oil should be changed before the contamination level reaches the point where engine damage can result. Because it is difficult for the individual motorist to determine when the contamination level is too high, automobile manufacturers provide recommended oil change intervals. These change recommendations vary by model year and manufacturer. 

Recommended intervals and mileage limits also vary with the type of service under which a car operates. More frequent oil changes are recommended for severe service.

Engineers work to establish an optimal viscosity for oil, based on load and speed conditions. They balance lighter – or low-viscosity – oil, which provides little resistance to motion thereby saving fuel and efficiently transferring horsepower, with a heavier – or high-viscosity – oil that resists being squeezed out of the contact area between metal surfaces. The complicating factor is that the viscosity of oil varies with changes in temperature – thinner when hot, thicker when cold. At low temperatures, we need the engine oil to flow readily (not thicken too much or gel). At high temperatures, we need the engine oil to keep from becoming too thin and allowing metal-to-metal contact. To counter this, engineers developed multigrade engine oils

Viscosity is a measure of a fluid's resistance to flow. A fluid with low viscosity flows easily and is often called "thin". Water is an example of a fluid with a relatively low viscosity. A fluid with high viscosity is often described as "thick". Maple syrup is an example of a fluid with a relatively high viscosity.

Engine oils are currently classified by a two-letter code. Gasoline engine oil categories start with the letter S (originally designated "Spark Ignition" engine oils, we now associate the S with "Service"). Diesel engine oil categories start with the letter C (originally designated "Compression Ignition" engine oils, we now associate the C with "Commercial"). The second letter is simply a sequential designation of improving quality levels over time. In other words, when a new industry quality level is established, the next letter of the alphabet is used (so SJ replaces SH). The letters "I" and "K" were purposefully skipped to eliminate potential confusion with other commonly used designations.