Today, motorists are driving vehicles where preventative maintenance is even more critical than ever before.

It is important for anyone who wants to get the maximum service from their vehicle to read the owner's manual carefully, particularly the preventative maintenance recommendations.  These recommendations should be followed.  If, by any chance, a customer has purchased a used vehicle and does not have an owner's manual, the manufacturer should be able to supply one.

Of course, one must understand what is being read in the owner's manual.  Over the years, the emphasis in most manuals has been extended service intervals, in order to make the vehicle as "maintenance-free" as possible.  It is for this reason the preventative maintenance schedules should be followed to the letter, and the vehicle owner understands the differences in service requirements.

For instance, for a number of years, most new vehicle owners' manuals have recommended oil changes every 7,500 miles (12,000 km) or 12 months, and oil filter changes at the first oil change and thereafter at every second oil change, or at 15,000 mile (24,000 km) intervals for "normal" service.  For diesel and turbocharged engines, the owners' manuals recommend more frequent oil and filter changes.

For "severe" service, the owners' manuals recommend an oil change every 3,000 miles (5,000 km) and an oil filter change with every oil change.  This is the interval also recommended by the Filter Manufacturers Council and the American Petroleum Institute.

However, what the owners' manuals call "normal" operating conditions are really ideal conditions, mostly long highway trips rather than around-town driving.  According to a survey done by a member company of the Filter Manufacturers Council, only about 20 percent of vehicles are regularly driven under "normal" conditions.  That means that 4 out of 5 drivers are in the "severe" driving category.  But no one likes to think of themselves as a severe driver.  You might want to call the two categories "ideal" and "average" rather than the "normal" and "severe" titles used in the owners' manuals.  Since 80 percent of motorists drive under less than ideal conditions, the "average" title is certainly justified.

Severe (or average) service, as defined in most owners' manuals, includes trips of less than four miles (6 km), particularly in cold weather.  Under these conditions, the engine does not warm up completely, causing moisture to accumulate in the crankcase.  Every time the engine is started, the oil is contaminated with blow-by soot, raw gasoline and condensed moisture.  Unless the engine gets regular extended highway travel, these contaminants remain in the oil.

After 3,000 miles (5,000 km) of "severe" - or "average" - driving, the oil can be saturated with foreign substances.

Within the same time period, the additive package may be depleted.  The additives account for 20 percent of each can of premium quality oil, and include various chemical detergents, dispersants, anti-oxidants, viscosity improvers, and corrosion inhibitors.  Although the oil does not wear out, the additives do.  Unless they are replenished, the oil cannot properly function in today's internal combustion engines.

Trips of less than four miles (6 km) is the key factor in keeping most motorists in the average or "severe" category. In most owner's manuals the following are considered "severe" or average conditions:

Frequent idling for long periods of time, such as stop-and-go driving in heavy traffic.  Many vehicles are subjected to this condition twice a day in commuting to major cities.

• Sustained highway driving in hot weather, such as vacation travel.
• Towing a boat or trailer, carrying heavy objects on a rooftop rack.
• Driving in dusty conditions, such as over dirty or gravel roads, or where volcanic ash is blowing around.
• Prolonged operation at sub-zero temperatures.
• Driving on steep hills or mountains on a regular basis.