6. Severe service includes: short-trip driving, driving in dusty conditions, heavy traffic, cold weather, hot weather, mountainous terrain, and towing trailers, boats, and other items. Most motorists are surprised to learn that one example of severe-service driving is the legendary little old lady who only drives her car a few miles to church on Sunday. But it's true. She should change her oil more often than the long-trip, highway-speed driver. The problem with short-trip driving, driving in heavy traffic (stop-and-go driving), and cold-weather driving is that the engine never warms up enough to boil away the water or fuel contaminants that form in the oil .

Water forms (as steam) during the combustion process and condenses on the interior surfaces of a cold engine. When it combines with the nitrogen oxides and sulfur oxides in exhaust, water forms acids that corrode engine surfaces and promote engine wear. Water in the engine also promotes sludge formation. Sludge is a thick, mayonnaise-like compound formed by the interaction of oil, gasoline, water, and combustion by-products. It obstructs oil passages and restricts oil supply. Built-up sludge eventually will bake on the engine surfaces, hampering operation.

In addition, until the engine warms up, temperatures in the combustion chamber aren't high enough to burn the gasoline completely. If gasoline leaks past pistons, it dilutes the oil, reducing its lubrication and protection capacity.

Driving in dusty conditions also causes problems, since no air cleaner is 100 percent effective. Dust grinds away at metal parts in your engine and combines with oil to create sludge. The same thing happens if you drive on roads that have been salted or sanded for icy conditions. Hot-weather driving is considered severe service because it can cause oil to oxidize, thickening it so it cannot flow through the engine efficiently. Towing heavy items also encourages oil oxidation - as does driving in mountainous terrain. In addition, if your engine is out of tune, it won't burn fuel efficiently. The unburned fuel will dilute the oil and form engine sludge.

As you can see, "normal service" is long-trip and highway-speed driving on paved roads during good weather with light loads. If most of your driving occurs under these conditions, you can follow the "normal service" oil- and filter-change schedule in your owner's manual. However, if - like most modern drivers - you do a lot of short-trip, stop-and-go driving, follow the "severe service" guidelines and change your oil and filter every three months or 3,000 miles - whichever comes first.