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Fluid selection

Fifty years ago, selecting fluids for automotive service was pretty simple. Motor oil was rated as ML, MM or MS, for motor light, moderate or severe, respectively. The earliest automatic transmissions operated on motor oil. For brake fluid, gear oil and power steering fluid, we were pretty much limited to one choice each.

While chemical engineering and fluid technology improved steadily over the decades, fluid selection stayed pretty basic through the 1970s. A couple of new types of ATF were developed, brake fluid specifications-DOT 3 and DOT 4, for example-emerged and the "S" and "C" service classifications for motor oil showed steady improvement in lubricants. Then in the 1980s and '90s, fluid diversity erupted.

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Today, the automotive service industry is faced with a daunting variety of fluids. One type certainly does not fit all applications. This article summarizes some of the most common fluids and the unique OEM specifications that have appeared over the last 10 to 15 years and that are in use today.

Motor Oil

Motor oils are classified by Society of Automotive Engineers (SAE) viscosity ratings and American Petroleum Institute (API) service classifications. The viscosity rating indicates how easily an oil flows at a specific temperature. The lower the number, the more easily it flows. Oils tested for viscosity at low temperatures have a "W" (for winter) after the number; oils tested at high temperature have no letter after the viscosity number. Most oils sold today are tested and classified for viscosity at both high and low temperatures. These multiviscosity oils have designations such as 5W-30 or 20W-50.

API service classifications were introduced in the late 1960s to indicate oil resistance to wear, oxidation, corrosion and deposit formation, as well as its overall ability to lubricate. Oils for gasoline engines have service classifications beginning with an "S" (for Service) and followed by a letter "A" through "L" to indicate increasing performance levels. Oils for diesel engines have service classifications beginning with "C" (for Commercial) and followed by a letter "A" through "I."

When the "S" and "C" service classifications were introduced, they were a logical way to define motor oil performance requirements and the evolution of those requirements. The system went along pretty well for about 20 years-through the 1980s-and then diversity set in.

Besides a service classification, some oils for the past dozen years also are designated as "energy-conserving" or "energy-conserving II." These oils reduce engine friction and improve fuel economy. Energy-conserving oils have more faction-reducing additives and relatively low viscosities.

Since 1993, oil quality also is identified by the API "starburst" symbol. To earn the starburst, oils must meet the energy-conserving II requirements of the International Lubricant Standardization & Approval Committee (ILSAC) and be certified as the correct oil for gasoline engines in cars and light trucks. Therefore, only multiviscosity oils qualify. Oils that qualify for the starburst symbol also may carry ILSAC classifications GF-1, GF-2 or GF-3, which indicate the energy-conserving qualities.

The energy-conserving properties of multiviscosity oils have been the driving force in oil development, but strict emissions regulations also have kept engineers busy. Current regulations have lowered allowable hydrocarbon (HC) emissions to almost immeasurable levels. HC tailpipe emissions are primarily unbumed gasoline. Reducing HC emissions means that engineers must eliminate anyplace in the combustion chamber and cylinder where a molecule of gasoline might hide.

Piston-to-cylinder clearances have been reduced to .001 inch or less. Piston ring gaps have been reduced, and piston skirts have been coated with friction-reducing synthetic materials. Motor oil for modem engines must have low viscosity at startup to get the oil into those tight clearances fast. Thus, many carmakers recommend 5W-30 oil as the preferred viscosity. If your customers question these recommendations, explain the engineering background. High-viscosity oil in a late-model engine may do more harm than good.

Automatic Transmission Fluid

The first automatic transmissions of 65 years ago operated on motor oil that was dyed red to help identify leaks. Automatic transmission fluid (ATF) has since evolved into one of the most complex lubricants in the oil industry.

When engineers design an automatic transmission, they consider the frictional properties of the ATF. All ATF types fall into two groups-(hose that contain friction modifiers and those that don't. Friction-modified ATF reacts to lower the coefficient of static friction as a clutch or band locks up. This creates a very smooth shift. Unmodified ATF provides a higher coefficient of friction as the clutch or band locks up and creates a firmer shift.

The first friction-modified ATF was Type A, introduced as a GM spec in the mid-'50 s. It evolved to Type A, Suffix A and eventually to Dexron fluid, introduced by GM in 1967. The first unmodified ATF was Type F, introduced by Ford in 1959 and used in all Ford transmissions through 1976 (and severed after that date).

Automotive Industry

Fluid selection

Motor, Nov 2003 by Layne, Ken

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