However, many people think the oil should be changed more often. To maintain the vehicle's warranty, you need to maintain the manufacturer's recommended oil change interval. But different kinds of driving may require more frequent changes. For example, for that very same car, the owner's manual instructs an oil-and-filter change interval of 3,750 miles or six months if it's being operated under severe conditions, which are defined as:
> Driving less than five miles per trip, or less than 10 miles per trip in freezing weather
> Driving in temperatures more than 90 degrees Fahrenheit
> Extensive idling or long periods of stop-and-go driving
> Towing a trailer, using a car-top carrier or driving in the mountains
> Driving on muddy, dusty or de-iced roads
Notice what's first: Short trips, particularly in cold weather. Engineers will tell you that that's just about the harshest duty cycle for engine lubrication. Consider this: In the morning, start it up and go five miles to work, then shut if off. Four hours later, for lunch, start it up, go two miles to a restaurant, then shut if off. About one hour later, start it up, go two miles back to work, then shut if off. Another four or five hours later, start it up, go five miles to home, then shut it off. It's been driven 14 miles, never once given a chance to get fully warmed up, and had four cold starts. The oil has never been able to get rid of the moisture that forms from condensation or the residues of combustion processes that would be burnt off with heat, everything turns to a thick sludge, and if you don't give it more frequent oil changes you're in for expensive trouble. That sludge can eventually clog the oil pump screen, lubrication gets shut off, and a connecting rod ends up going through the side of the engine block. Buying a new engine gets kind of expensive.
On the other hand, a guy who starts up the car, hits the freeway and doesn't stop for 200 miles has done a lot less harm. His engine has had only one cold start and plenty of time to get everything fully warmed up and working properly.
What about those expensive oils that are claimed by their manufacturers to deliver much longer change intervals, such as 25,000 miles? Maybe they'll deliver, but remember this: If you don't change oil at the manufacturer's recommended interval, and you have a problem, you may have a lot of trouble getting satisfaction on the warranty. If you drive in normal conditions, there is little to recommend paying extra for that high-priced oil, since you have to change it at the regular interval anyway.
However, if you live where it's really cold in the winter, we're talking Alaska or such, there is some evidence that synthetic oil may provide more protection for those brutal January morning start-ups.
And if you have an older car -- say one of those old classics or musclecars -- you may want to consider twice before using synthetic oil. Some of the seal and gasket materials used on older cars are not compatible with synthetic oils, and you may end up with a bunch of leaks all over the garage floor.
It goes without saying that you should consult your vehicle's owner's manual for the correct viscosity and grade of oil, and purchase oil from a reputable, known manufacturer. Don't scrimp on the filter, either. It is, after all, your car.