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Name: Stuart
Status: Student
Grade: 9-12
Location: MA
Country: United States
Date: July 2006

Does using synthetic motor oil lessen auto emissions? Is it worth the extra cost? If you were to use it, how often would you change the oil and filter?

Hi Stuart,

Syntetic oil will do nothing whatsoever to lessen auto emissions, as compared to a high quality standard motor oil. In any properly working motor, the oil never gets anywhere near the combustion process, and it is combustion that creates the emissions.

The main advantage of synthetic oil is that it is slower to break down than normal oil, and therefore the time between oil changes can be longer. Also, some of the chemical breakdown that oils suffer from, is not the oil itself breaking down, but their additives such as "viscosity index improvers" degrading. Synthetic oils need less of these additives, so they suffer less from their breakdown.

How often should you change the oil and filter if one uses synthetic oil? You ALWAYS change according to the vehicle manufacturer's recommendations in the owners manual. Since each manufacturer has specific instructions, I cannot give you a general answer. As a rule of thumb, it is often considered safe to extend the oil change interval by 50% with synthetic oil, but again, you do what the vehicle manufacturers says to do!

As for the filter, it should be changed at the same interval as when normal oil is used. The filter's job is to filter out abrasive particles that mainly result from the combustion process. The type of oil has little to do with the formation of this particulate matter, so switching to a synthetic oil will not do anything much to reduce amount of particulates that the filter has to filter out.

Now, is synthetic oil worth it? Perhaps. Synthetic oils do not protect the engine better than normal oils. They just last longer. So if, for example, a synthetic oil lasted 50% longer than a high quality normal oil, and cost only 30% more, then I would say it would be worth it in that case. However, often, this kind of analysis shows synthetic oil cost more in the long run.


Bob Wilson.

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