When motor oil isn't changed often enough, varnish deposits and sludge begin to form inside the engine. If the oil is neglected long enough, it will eventually turn into goo and ruin the engine.
"Oil sludging" has become a serious problem in recent years for a number of reasons. One is that more new car buyers lease their vehicles rather than purchase them. Because they know they're only going to keep the vehicle for two, three or four years, they tend to forget important things like changing the oil and filter. They figure the next owner can pay the price for their neglect. But all too often, the oil doesn't go the distance and their engine is gunked up or ruined before the lease expires.
Aggravating the situation are consumer misconceptions about how often the oil needs to be changed. 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 more than a thousand complaints about oil sludging problems from motorists who thought they were following the service intervals recommended in their owner's 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. Worse yet, some vehicles have no scheduled oil change interval and use an "oil change reminder light" to signal the driver when an oil change is needed. Unfortunately, the reminder light may not come on until 12,000 miles or more in some vehicles - and even then the driver may ignore it.
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, whichever comes first. Why? Because this provides the best all-round protection for the average driver.
Most drivers should actually follow a "severe" service maintenance schedule (3,000-mile oil changes) rather than a "normal" service schedule to protect their engines. Severe service is when most trips are less than four miles (10 miles when the temperature is below freezing), prolonged driving at high speed during hot weather, idling for extended periods of time (20 minutes or more), towing a trailer and/or 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.
When the oil remains in the crankcase too long, the additives that enable it to lubricate and protect the engine become depleted. The oil wears out and no longer has the same viscosity range it had when it was new. It also gets dirtier and dirtier as moisture, acids, soot and wear particles accumulate. The filter can only remove so much, and eventually plugs up allowing unfiltered oil to reach the bearings. Once this occurs, wear skyrockets and the engine is headed for disaster.