Q Mechanics I’ve asked all tell me that motor oil should be changed every 3,000 miles, but my Audi’s manual says 8,000 miles. And I have read of a study with taxis that found there was no greater engine wear with 10,000-mile oil change intervals than with 3,000-mile intervals. What’s the story?

LARRY LUSVARDI   
   Crystal Bay, Nevada  

A Our Car Care Plus technicians favor 3,000-mile/three-month intervals on the grounds that it’s cheap insurance against wear. It also serves as a safety measure: The mechanic who changes the oil can check your car’s overall condition at the same time, catching potential troubles before they grow. If you do follow the owner’s manual, use its most conservative schedule, the one for "severe service." In the eyes of carmakers, the ordinary stop-and-go daily driving that most of us consider normal is severe service. As for taxis, they’re not driven the way family cars are; once they’re turned on they run all day or all night, so they don’t get the repeated cold starts and condensation that can dilute oil. Over time, oil components can break down, so it’s a good idea to observe the three-month time interval even if your vehicle has not yet reached the mileage figure given in the owner’s manual.

Q I have a 2003 Chevy S-10 pickup truck with a "clutch fan." What the heck is a clutch fan and what is its purpose?

TOM MCLAUGHLIN   
   Soulsbyville, California  

A The speed of the fan you can see facing the radiator in most cars used to be directly related to engine speed, as the fan was driven by a belt attached to the engine’s crankshaft. This setup doesn’t necessarily provide the most cooling when the engine needs it, and it can have the fan doing its darnedest when a more laid-back approach would be better. Clutch fans are more responsive to the engine’s need for cooling; they’re thermostatically controlled, so they spin faster or slower depending on the engine’s actual temperature, as communicated by heat sensors.

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Please send car problems to Your Car, VIA, 150 Van Ness Ave., San Francisco, CA 94102. Send email to viamail@csaa.com. Questions will be answered only in the magazine.