Oil is a vital component of engines. Engines are made of very carefully assembled metal parts that must work quickly and precisely in order to run the engine. Oil lubricates these parts and allows them to last longer by providing a buffer between the different parts of the engine that move together. Without these lubricants, the metal parts would grind together and wear down the engine. Over time, contaminants can build up in the car oil, until it needs to be replaced in order to continue doing its job.
A high mileage oil is simply an oil designed for use in older engines that have already experienced a significant amount of wear. After driving the car so many miles, these engines may be more subject to problems than newer engines. When you look for oils, look for labels that say "high mileage" or indicate that they are for older engines. These include two additives not found as often in normal car lubricants.
Burn-off inhibitors are extra additives designed to make it very difficult for oils to combust if they reach any of the combustion chambers in the engine. Combusted oils leave behind residues that can build-up in the combustion chamber when they burn and cause problems for the engine over time.
Seal conditioners are additives designed specifically to help seal small leaks in gaskets. These chemicals are designed to soak into the rubber of the gasket seals and cause them to expand. In theory, this helps to seal spaces that could develop in older engines due to wear and tear. While the seal conditioners may not solve larger leak problems, they can slow down the rate at which oil escapes.
There is some doubt that these additives are completely effective in helping older engines maintain their integrity or improve their functions. The additives that are already added to normal engine oils have been proven to extend the lives of engines. It is not likely that extra additives would improve engines to a significant degree, or they would be used in all engine oils. Mechanics debate whether high mileage oils are worth the extra cost or not.
How often oil should be changed in a high mileage engine depends on the type of engine and how it is used. High mileage oil is commonly used in cars with 75,000 miles or more on the odometer, but are not necessarily needed if the engine is running well. If oil leaks are evident, or the car's exhaust has turned dark or sooty, then it may be time to change to a stronger oil.
