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My wife's car often uses to drive to a nearby grocery store, which is too close to our house (about 2 miles). Some one told me that I should change the engine oil more often because engine is working hard for short drive. I don't know why.

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Cold Starting Is Bad for All Engines At cold, oil flow is insufficient to properly lubricate the engine because the oil is too thick; in fact, no oil available with today's technology will provide sufficient flow at cold and also be able to protect the engine at operating temperature. The choice, then, is to pick an oil that will perform poorly at start up. As a result, starting an engine cold is pretty much the worst normal operating procedure you can inflict on it. Short drives mean more cold starts as a percentage of miles travelled, so there will be relatively more wear on the engine.

Condensation Does Not Evaporate A short trip will not allow sufficient time for condensation in the oil system to burn off, which may result in a build-up of residue on your oil cap and other cooler parts of the system. This is not harmful, strictly speaking, but it may make it more difficult to diagnose other problems that would result in similar symptoms (e.g., head gasket failures).

Engine May or May not Be Working Harder I'm not convinced that in a modern engine moving oil around, even cold, is going to result in much extra strain on the engine or oil itself, unless we're talking about extreme cold conditions, in which case you must use synthetics because conventional oils will actually stop flowing altogether.

What Oil Change Interval to Use Since your mileage will, presumably, be low, you will be changing your oil based on time, not distance. This is important to do because oil thickens over time (even in storage), exacerbating the cold start issue. Six months is considered standard and it's highly unlikely, under any kind of normal use pattern, you'd want to change it more frequently than that.

What Kind of Oil to Use Synthetics thicken less than conventional oils, both when cold and in storage, so a synthetic would be a better choice in your application.

Conclusion Synthetics are more expensive and frequent oil changes are expensive. The reality is, if you don't have a very nice engine that you want to pamper, none of the above is particularly relevant because the short drives won't likely have a measurable impact on your engine's longevity. So you have to decide if it's worth the cost. If you're talking about a race-tuned 4A-GE that you want to keep forever, then change the oil every six months with a carefully-selected synthetic (but then, if that's the case, you already know all this). If you have a 1ZZ-FE that you're not particularly attached to, then I really wouldn't worry about the short trip issue at all. Either way, the best way to know if you're doing this right, with regard to the oil, is to send in your oil for a used oil analysis, but again, depending on your circumstances, you may not want to bother.

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Welcome to the site! –  Mark Johnson Dec 6 '12 at 18:31
    
As addition to the "Condensation Does Not Evaporate" point: This is also valid for water vapour in the exhaust. When a car is used for a short trip the muffler will not heat to the point where water turns into steam. As a result it tends to rust quicker. –  Hennes May 6 '13 at 21:45
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If you stick to 3 months or 3000 miles, you should be fine. But short trips don't give the oil time to circulate or heat up and thin out. Contaminants don't get removed as well... Not to mention that the difference in viscosity of hot versus cold oil is significant, and yes, that means the motor works harder to churn through it as well as pump it throughout the engine

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Most notably it's far too short a drive for the engine get fully to temperature and burn out all the moisture. –  Brian Knoblauch Dec 5 '12 at 17:10
    
Yes another good point –  Drake Clarris Dec 5 '12 at 18:58
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It is also really tough on exhaust systems as moisture tends to collect in the muffler and low spots of the system. It is even worse when the muffler is at the end of the system as it takes a long time for the muffler to get hot. –  mikes Dec 6 '12 at 0:04
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