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.

EXTENDED OIL CHANGE INTERVALS

In recent years, many vehicle manufacturers have extended their recommended oil change intervals to reduce maintenance costs for the vehicle owner -- and have run into trouble. The Center for Auto Safety (www.autosafety.org) has logged over a thousand complaints about oil sludging problems from motorists who thought they were following the service intervals recommended in their owners manuals but ended up with a crankcase full of sludge.

Extended oil change intervals of 7,500 or 10,000 miles or more are based on ideal operating conditions, not the type of short trip, stop and go driving that is typical for many motorists. Consequently, most drivers should follow a "severe" service maintenance schedule rather than a "normal" service schedule to protect their engines.

Severe service includes:
* Most trips are less than 4 miles.
* Most trips are less than 10 miles when outside temperatures remain below freezing.
* Prolonged high speed driving during hot weather.
* Idling for extended periods and continued low speed operation (as when driving in stop-and-go traffic).
* Towing a trailer.
* Driving in dusty or heavily polluted areas.

Some engines, such as diesels, suffer more blowby than others and typically require more frequent oil and filter changes. For most passenger car and light truck diesels, the oil should be changed every 3,000 miles without exception -- especially in turbo diesels.

Turbocharged gasoline engines also require more frequent oil changes because of the high temperatures inside the turbo that can oxidize oil. A 3,000 mile oil change interval is also recommended for all turbocharged gasoline engines.

OIL REMINDER LIGHTS

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.

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).