Dozens of different motor oil formulations are available on today's market, and each claims that it's a better value than the others. In a sense, they are all correct. Modern engine oil is very different from the lubricants of old; new formulations start from more refined base stocks and contain a number of sophisticated additives to increase performance, mileage and engine life.
Engines continually shed microscopically thin layers of metal from their bearings and rubber from their internal seals. The only way to slow this wear is to introduce a boundary layer of lubricant between the components, but even the best lubricants can't stop wear entirely. As the engine's components wear out, the clearances between them increase and may allow some oil to slip past and escape the engine or burn inside its cylinders.
Most high-mileage engine oils are sort of a halfway point between a synthetic blend and a conventional oil. Realistically, all synthetic blends use a combination of conventional (mineral) oil, machine-grade mineral oil and pure synthetic lubricant (defined as a lubricant produced in a lab, as opposed to one refined from petroleum). High mileage oils generally use a bit less pure synthetic oil--which costs about £260 a quart in its pure form--and a bit less conventional oil than a synthetic blend. By volume, high mileage oil is primarily high grade mineral oil.
High mileage engine oil contains several different types of additives designed to prevent leaks and prolong engine life. Its primary additive is a "seal conditioner" that not only soaks into the engine's valve seals and lubricates them, but actually causes the seals to expand. After expansion, the valve seals press tightly against the valve stems, which helps to reduce the amount of oil entering the cylinders.
Some high-mileage oils contain a powdered compound called CSL, which stands for Copper-Silver-Lead. When introduced into a hot engine, CSL powder melts and serves to fill in the tiny pits in cylinder walls, piston rings and valve stems. These small pits allow oil to enter the combustion chamber and hot engine gases to leave it, and can contribute to accelerated engine wear by scuffing away before the lower-lying portions of the cylinder wall and piston rings.
Engines that require high mileage engine oil are usually those that already exhibit signs of wear, such as excess oil consumption, oil smoke (blue smoke coming from the exhaust pipe), engine blowby (hot exhaust gases coming from the valve-cover breather), a loss of power and/or fuel economy. If your engine has more than 75,000 miles on it and you haven't been running synthetic oil since day one, there's a fair chance it has significant internal wear. High mileage engine oil can help to reduce such wear-related symptoms, so it might be worth a shot if you've never used it before.
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