Dirty oil just doesn’t do the job as well as fresh oil does. The additives in dirty oil boil out, contaminants form in the crankcase and eat metal parts, and water collects over time and forms sludge. The oil holds more and more abrasive particles of metal suspended in it, and these particles wear away the parts of the engine that the oil is supposed to protect. That’s why you should change your oil at regular, reasonable intervals. But what is a reasonable amount of time — or mileage — between oil changes?
Tip: All oil looks pretty black within a couple of days after an oil change, so the only way to avoid running on oil that’s so dirty that it becomes a liability is to keep a record of when it was last changed and to change it frequently — as often as every 1,000 miles in extreme operating conditions (I get to what those are in a minute). By changing your oil frequently, you may get twice the mileage out of an otherwise good engine.
Car Smarts: You’ll be surprised by what some "extreme operating conditions" are: If you do a lot of stop-and-go driving in city or rush-hour traffic, make a lot of short trips each day and leave the car parked long enough to have your engine cool down between them, and don’t often get up to high speeds on a highway, your engine rarely gets hot enough to evaporate the water that forms in the crankcase and builds up sludge in the engine. Other extreme conditions are if you drive in very hot weather or in areas with a lot of blowing dust or dirt, or tow or haul heavy loads all the time. In any of these circumstances, change the oil as often as every 1,000 to 3,000 miles on older vehicles. On new vehicles, follow the manufacturers’ recommendations for extreme use.
Some manufacturers suggest that oil be changed every 7,500 miles or more, but that’s based on optimum operating conditions, and the manufacturers are the ones who get to sell you a new vehicle if your old one wears out prematurely. Although new vehicles can run longer on the same oil than older ones can, and improvements in motor oil have extended its efficiency over longer periods of time, to be on the safe side, I now change my oil every 5,000 miles or every six months, whichever comes first. If you’re a freeway driver who goes on a lot of long journeys at high speeds, you can probably extend the oil change interval, but on anything but the newest vehicles I wouldn’t go longer than 5,000 miles between changes. And never, under any circumstances, go farther than the manufacturer’s recommended maximum interval between oil changes.
From Auto Repair for Dummies, copyright © 2009 by Wiley Publishing, Inc., Indianapolis, Indiana. Used by arrangement with John Wiley & Sons, Inc.