But don't assume that if a synthetic is so good (read: very expensive) that you don't need to change it as often. The base lubricant may well be better, but the additive package, which can be as much as 25 percent of the volume of product in the bottle, can still become exhausted. And unburned fuel, partially burned hydrocarbons, atmospheric dirt, metal wear particles and blowby carbon particles will build up just as fast in a synthetic-lubricated engine as in one laved in petroleum-based oil. The only way to remove this stuff is to drain and replace the oil.