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When to Change Your Oil

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Is It Mileage or Time That Determines When to Change Your Oil?
The rule of thumb for oil changes is every 5000 -7500 miles, depending on the manufacturer's recommendation. But what if you do not drive more than 7500 miles for the entire year? If you use the car only for short trips on city streets, particularly in cold weather, you probably should have the oil changed every three months.

Such driving, in which the engine never reaches its proper operating temperature, can cause condensation of water inside the crankcase and dilution of the oil by gasoline.

Water contamination of the oil occurs when moist air is drawn into the crankcase and condenses after the engine is turned off. Frequent short trips increase the amount of condensation.

The water in the crankcase is not harmful, but it can combine with sulfur, a byproduct of combustion, to form sulfurous acid, a weak acid that breaks down the lubricating qualities of the oil.

The fuel contamination is a separate problem. In the carburetors of older engines and even some fuel-injected engines, a richer mix of fuel is sent to the engine on cold starts. Also on cold starts, some gasoline seeps down the cylinder walls into the crankcase. A small amount of gasoline contamination is not harmful, but larger amounts dilute the oil and lower the viscosity-again compromising the oil's lubricating qualities.

Another problem associated with driving a car only on short trips is that engine deposits tend to increase because the combustion chamber never heats up enough to burn off hard carbon that forms on the piston head and valves. Eventually, this can cause engine ping, because the deposits create localized hot spots inside the combustion chamber that cause the fuel to burn unevenly.

When you drive a car a long distance, the engine, coolant and oil all get quite hot and these contaminants are boiled out of the oil and soot does not form as quickly. If the weather is cold, you need to drive a lot farther, certainly more than 10 miles at freeway speeds, to reach operating temperature.

If you drive mostly on long but infrequent trips, there is nothing wrong with changing your oil every six months or even every year, according to Texaco oil experts.

The final consideration is your car warranty. If the manufacturer specifically requires you to change the oil based on elapsed time, it would probably be worth doing so until the warranty expires. If the engine needed a repair covered under the warranty, a dealer might balk at honoring a claim if the oil changes did not follow the manufacturer's recommendations.

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