The oil and filter in an engine normally should be changed at 3,000-5,000 mile intervals, reminds the Car Care Council. Oil that stays in an engine beyond its life expectancy is abrasive, corrosive and can clog tiny orifices in the system.
Synthetic oil is another story.
The word synthetic as it applies to oil refers to the combining of chemicals as opposed to the refinement of crude oil found in nature. The processes themselves speak volumes about the two lubricants. The man-made one is designed while the other is refined. Synthetic oil contains a uniform molecular construction that has been created to perform consistently under the harshest circumstances. On the other hand, even after crude oil has been refined its molecules are of varying sizes.