Automotive Systems

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Engine Oil

ENGINE OIL

Engine oil, also called motor oil, is used to produce a lubricating film on the moving parts in an engine. The military specification for this type of oil prescribes that the oil shall be a petroleum or synthetic petroleum product or a combination thereof. This oil is intended for lubrication of internal-combustion engines other than aircraft engines or for general-purpose lubrication.

Oil Viscosity and Measurements
Oil viscosity, also called oil weight, is the thickness or fluidity (flow ability) of the oil. A high viscosity oil is very thick and resists flow. A low viscosity oil is very thin and flows easily.

Oils are graded according to their viscosity by a series of Society of Automotive Engineers (SAE) numbers. The viscosity of the oil increases progressively with the SAE number. An SAE 4 oil would be very light (low viscosity) and SAE 90 oil would be very heavy (high viscosity). The viscosity of the oil used in internal-combustion engines ranges from SAE 5 (arctic use) to SAE 60 (desert use). It should be noted that the SAE number of the oil has nothing to do with the quality of the oil.

The viscosity number of the oil is determined by heating the oil to a predetermined temperature and allowing it to flow through a precisely sized orifice while measuring the rate of flow. The faster an oil flows, the lower the viscosity. The testing device is called a viscosimeter. The viscosity of the oil is printed on top of the oil can. Oil viscosity is written SAE 10, SAE 20, SAE 30, and so on. The letter W will follow any oil that meets SAE low-temperature requirements. An example would be SAE 10W.

Multi-viscosity oil or multi-weight oil has the operating characteristics of a thin, light oil when cold and a thicker, heavy oil when hot. A multi-weight oil is numbered SAE 10W-30, 10W-40, 20W-50, and so on. For example, a 10W-30 oil will flow easily (like 10W oil) when starting a cold engine. It will then act as a thicker oil (like 30 weight) when the engine warms to operating temperature. This will make the engine start more easily in cold weather. It will also provide adequate film strength (thickness) when the engine is at full operating temperature.

Normally, you should use the oil viscosity recommended by the manufacturer, However, in a very cold, high mileage, worn engine, higher viscosity may be beneficial. Thicker oil will tend to seal the rings and provide better bearing protection. It may also help cut engine oil consumption and smoking.

Oil Service Rating
The oil service rating is a set of letters printed on the oil can to denote how well the oil will perform under operating conditions. The American Petroleum Institute (API) sets this performance standard.

The API system for rating oil classifies oil according to its performance characteristics. The higher rated oils contain additives that provide maximum protection against rust, wear, oil oxidation, and thickening at high temperatures. The oil service ratings are as follows:

  1. SA—adequate for utility engines subjected to light loads, moderate speeds, and clean conditions. Contains no additives.
  2. SB—adequate for automotive use under favorable conditions (light loads, low speeds, and moderate temperatures) with relatively short oil change intervals. Generally offers only minimal protection to the engine against bearing scuffing, corrosion, and oil oxidation.
  3. SC—meets oil warranty requirements for 1964 through 1967 automotive gasoline engines.
  4. SD—meets oil warranty requirements for 1968 through 1970 automotive gasoline engines. Offers additional protection over SC oils that are necessary with the introduction of emission controls.
  5. SE—meets oil warranty requirements for 1972 through 1979 automotive gasoline engines. Stricter emission requirements created the need for this detergent oil.
  6. SF—meets oil warranty requirements for 1980 through 1988 automotive gasoline engines. The SF oil is designed to meet the demands of small, high-revving engines. A SF oil can be used in all automotive vehicles requiring detergent oil.
  7. SG—meets oil warranty requirements for 1989 through present automotive gasoline engines. Contains more additives than SF oils. Can be used as CC or diesel type oils. It is a detergent oil.
  8. CA—meets all requirements for naturally aspirated diesel engines operated on low sulfur fuel.
  9. CB—meets all requirements for naturally aspirated diesel engines operated on high sulfur fuel.
  10. CC—meets all requirements for lightly supercharged diesel engines.
  11. CD—meets all requirements for moderately supercharged diesel engines.

The operator's manual provides the service rating recommended for a specific vehicle. You can use a better service rating than recommended, but NEVER a lower service rating. A high service rating (SG, for example) can withstand higher temperatures and loads while still maintaining a lubricating film. It will have more oil additives to prevent oil oxidation, engine deposits, breakdown, foaming, and other problems.

Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

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