Motor oil does more than just lubricate an engine. It also forms a film on bearing surfaces that lifts and separates moving parts so they don't touch to reduce friction and wear. The oil film also acts like a shock absorber to cushion reciprocating and rotating parts. Oil also serves as a coolant for critical engine parts such as the crankshaft bearings and valve-train. Oil also helps prevent rust and corrosion inside the engine, and helps keep surfaces clean by dissolving and carrying away dirt and varnish deposits.
Engine Lubrication System

UNDERSTANDING VISCOSITY

"Viscosity" refers to how easily oil pours at a specified temperature. Thinner oils have a water-like consistency and pour more easily at low temperatures than heavier, thicker oils that have a more honey-like consistency. Thin is good for easier cold weather starting and reducing friction, while thick is better for maintaining film strength and oil pressure at high temperatures and loads.

The viscosity rating of a motor oil is determined in a laboratory by a Society of Automotive Engineers (SAE) test procedure. The viscosity of the oil is measured and given a number, which some people also refer to as the "weight" (thickness) of the oil. The lower the viscosity rating or weight, the thinner the oil. The higher the viscosity rating, the thicker the oil.

Viscosity ratings for commonly used motor oils typically range from 0 up to 50. A "W" after the number stands for "Winter" grade oil, and represents the oil's viscosity at zero degrees F.

Low viscosity motor oils that pour easily at low temperatures typically have a "5W" or "10W" rating. There are also 15W and 20W grade motor oils.

Higher viscosity motor oils that are thicker and better suited for high temperature operation typically have an SAE 30, 40 or even 50 grade rating.

These numbers, by the way, are for "single" or "straight" weight oils. Such oils are no longer used in late model automotive engines but may be required for use in some vintage and antique engines. Straight SAE 30 oil is often specified for small air-cooled engines in lawnmowers, garden tractors, portable generators and gas-powered chain saws.

MULTI-VISCOSITY OILS

Most modern motor oils are formulated from various grades of oil so the oil will have the best characteristics of both thick and thin viscosity oils. Multi-viscosity oils flow well at low temperature for easier starting yet retain enough thickness and film strength at high temperature to provide adequate film strength and lubrication.

A thin oil such as a straight 10W or even a 20W oil designed for cold weather use would probably not provide adequate lubrication for hot weather, high speed driving. Likewise, a thicker high temperature oil such as SAE 30 or 40 would probably become so stiff at sub-zero temperatures the engine might not crank fast enough to start.

Multi-viscosity grade oils have a wide viscosity range which is indicated by a two-number rating. Popular multi-viscosity grades today include 5W-20, 5W-30, 10W-30, 10W-40 and 20W-50. The first number with the "W" refers to the oil's cold temperature viscosity, while the second number refers to its high temperature viscosity.

WHICH VISCOSITY TO USE?

The "service rating" of motor oils is classified by the American Petroleum Institute (API). The program certifies that an oil meets certain OEM quality and performance standards. The service rating is shown in the API "Service Symbol Donut" on the product label. There may also be an "API Certified for Gasoline Engines" seal on the label.

API also gives oils an "Energy Conserving" rating if the oil meets certain criteria for reducing friction and oil consumption, and improving fuel economy.

Motor oils that meet the current API SM rating may also meet the new International Lubricant Standardization and Approval Committee (ILSAC) "GF-4" specifications, which some European and Asian auto makers require.

WHY OIL NEEDS TO BE CHANGED

Regardless of an oil's API service rating or additive package, all motor oils eventually wear out and have to be changed (actually, it's the additives that wear out more so than the oil). As the miles add up, motor oil loses viscosity and gets dirty. The oil no longer has the same viscosity range it had when it was new, and it contains a lot of gunk (moisture and acids from combustion blowby, soot, dirt and particles of metal from normal wear). You can't really tell much about the condition of the oil by its appearance alone because most oil turns dark brown or black after a few hundred miles of use.

The oil filter will trap most of the solid contaminants, and the Positive Crankcase Ventilation (PCV) system will siphon off most of the moisture and blowby vapors -- if the engine gets hot enough and runs long enough to boil the contaminants out of the oil. Even so, after several thousand miles of driving many of the essential additives in the oil that control viscosity, oxidation, wear and corrosion are badly depleted. At this point, the oil begins to break down and provides much less lubrication and protection than when it was new. If the oil is not changed, the oil may start to gel or form engine-damaging varnish and sludge deposits -- and eventually cause the engine to fail!

Oil life depends on many factors including driving conditions (speed, load, idle time, etc.), environmental factors (temperature, humidity, airborne dirt), and engine wear. As a general rule, most experts still recommend changing the oil and filter every 3,000 miles or six months, which ever comes first. Why? Because this provides the best all-round protection for the average driver.

General Motors, BMW and some of the other luxury brands have done away with recommended oil change intervals altogether and now use an "oil reminder" light to signal the driver when an oil change is needed. Some technicians now refer to this as the "Replace Engine Soon" light because of the sludging problems that have resulted from extending oil change intervals too far. The oil reminder systems estimate oil life based on engine running time, miles driven, ambient temperature, coolant temperature and other operating conditions. On some of these vehicles, the light may not come on until 10,000 miles or higher! But keep in mind that most of these engines are factory-filled with higher quality "synthetic" oil -- so be sure to replace same with same when the oil on these engines is changed.

OIL SENSORS & OIL ANALYSIS

One of the arguments against changing oil at specific mileage or time intervals is that the oil may still be good. As long as the additive levels in the oil are adequate and the oil is not oxidizing, breaking down or contaminated with fuel or coolant, there's no need to change it. Oil reminder lights are better than mileage/time intervals in this respect, but the light is still a guesstimate that may or may not be accurate. The only way to know for sure when the oil really needs to be changed is to test it. A sample of oil can be sent to a lab for analysis, and the report can be used to establish a change interval that reduces unnecessary maintenance. Many fleets use oil analysis to dermine oil change intervals, but for the average motorist, oil analysis is too expensive and inconvenient. The cost of the oil analysis is almost as much as an oil change.

The best approach is to use a sensor to measure the condition of the oil. A new oil monitor sensor called Intellistick is now available for this purpose. The sensor replaces the dipstick, and uses a bluetooth transponder to broadcast the condition of the oil to a laptop computer PDA or even a bluetooth enabled call phone.

SYNTHETICS

Synthetic oils are oils that are refined to a much higher degree than ordinary oils. Synthetic oils are premium oils that generally have greater viscosity stability, lower pour points and can withstand higher operating temperatures. Synthetic oils improve cold starting, reduce friction, reduce oil consumption and improve fuel economy and performance -- but they typically cost about three times as much as regular motor oil.

Some suppliers of synthetic motor oils say the higher cost of the premium quality oil can be offset by extending oil change intervals. But this would depend on the operating conditions, age and condition of the engine.

Synthetic oils are a good upgrade for most engines, but are not recommended for breaking-in newly rebuilt engines.

NEWS UPDATE Oct 30, 2007: A new green synthetic motor oil made from beef tallow (animal fat) is now available as an alternative to petroleum-based oils. The new oil is 100% biodegradeable, non-toxic, and safe (you can even drink it!). Performance is similar to other synthetic oils but it costs a little less. The product is called G-OIL.

For more information about synthetic oils, try these links:

AMSOIL Synthetic Oil
Castrol Synthetic Oil
G-OIL Green Motor Oil (biodegradeable synthetic oil from beef tallow)
Mobil1 Synthetic Oil
Pennzoil Synthetic Oil
Quaker State Synthetic Oil
Redline Synthetic Oil
Royal Purple Synthetic Oil

BLENDS & SPECIALTY OILS

For motorists who want the benefits of a synthetic oil in a less expensive product, there are "synthetic blends" that mix 20 to 25% synthetic oil with conventional oil. Blends cost about a dollar a quart more than ordinary oil, and provide many (but not all) of the benefits of a full synthetic.

There are also oils that have special additive packages for specific applications such as large, heavy Sport Utility Vehicles (SUVs), turbocharged engines (extra anti-oxidants) and high mileage engines (extra viscosity improvers and anti-wear additives).

OIL CHANGE TECH TIPS

* Drain the oil while it is hot. Contaminants will be in suspension and drain more easily from the engine.

* Always replace the filter when changing the oil

* Wipe some oil on the filter gasket so the seal won't stick or tear.

* Hand tighten the filter about 1/2 to 3/4 of a turn after the gasket makes contact. Over-tightening may damage the threads or gasket, and make the filter difficult to remove the next time the oil is changed. Under-tightening may allow the filter to work loose and leak.

* If the oil is badly contaminated or sludged, the crankcase should be cleaned and flushed before the engine is refilled with oil.

* Always check the oil level after refilling the crankcase. Start the engine, then shut it off and check the oil level after several minutes. It should be at the full mark on the dipstick. Most engine hold about four quarts of oil, plus half a pint to almost a quart for the filter (depending on the size of the filter). Overfilling can cause oil foaming and leaks. Under-filling may cause a loss of oil pressure and engine damage!

* Dispose of your old used oil properly. Save it in a container and take it to an auto parts store or other facility that recycles oil. Do NOT dump it on the ground, down a storm sewer or anyplace else where it can contaminate ground water.