Among those products are high mileage oils, advertised as specifically developed for high mileage engines. Some of them are partially synthetic, but most established brands like Castrol, Mobil 1, Pennzoil, or Valvoline offer fully synthetic products which are generally superior.
Like all modern engine oils, their secret ingredients are the additives which in this case are designed to make engines with 75,000 miles (120,700 km) or more run smoother, reduce sludge buildup, and revitalize seals and gaskets. To what extent they do that depends on a variety of factors.
This type of oil has been widely used, and many have reported noticeable improvements. Still, you should not expect it to perform miracles with an engine that received inadequate maintenance, has mechanical damage, or lubrication system issues.
I have talked with many mechanics about these oils in the past three years, and they agree that sludge is visibly reduced in some engines with small amounts of buildup. But then again, a normal, high-quality full synthetic oil can have the same effect, especially if you change it more frequently than specified by the carmaker.
Another characteristic that is heavily advertised is its ability to revitalize seals and gaskets. Yes, these oils contain some additives and seal conditioners that cause them to swell and become more effective. How effective is determined by the extent of their wear, so it might do the trick with some of them. However, those that are damaged will need to be replaced.
Most people also neglect to check if the oil meets manufacturers specifications, a major omission that can cause more harm than good. Using an oil that wasn’t designed for your car’s engine can significantly increase wear, even if the label states otherwise. Don’t get swept up by the advertising, and make sure to check specifications first.
In conclusion, you should use this type of oil if you have a car with more than 75,000 miles (120,700 km) to mitigate further wear of its old yet fully functional, well-maintained engine.