Here's a breakdown:
What high-mileage oils claim to do:
* Reduce leaks: This is the primary claim. They often contain seal conditioners aimed at swelling and softening aged seals, thereby reducing leaks. This *can* work to some extent, but the effectiveness varies greatly depending on the severity of the seal damage. Minor leaks might improve, while significant damage likely won't be fixed.
* Improve engine performance: This is a less specific and often less substantiated claim. Some formulations might offer slightly better wear protection than conventional oils, but the difference is usually marginal.
* Increase engine life: This is a difficult claim to prove conclusively. While better sealing and slightly improved protection *might* contribute to a longer engine life, other factors like overall maintenance significantly outweigh the impact of the oil.
What the evidence suggests:
* Seal conditioning: There's some evidence that the seal conditioners in high-mileage oils can help with minor leaks, but the effect is often temporary.
* Wear protection: High-mileage oils usually meet the same industry standards for wear protection as conventional oils of the same viscosity grade. The added benefits are often subtle.
* Overall impact: There is limited conclusive evidence to suggest high-mileage oils dramatically extend engine life beyond what good maintenance and using a properly specified oil would achieve.
In short:
High-mileage oils *might* offer some benefits for older engines with minor leaks, but they're not a miracle cure. The improvements are often modest and may not justify the sometimes higher price. Proper maintenance, regular oil changes using the correct viscosity and type of oil specified for your vehicle (as indicated in your owner's manual) are far more important for engine longevity. If you have significant leaks or other engine problems, high-mileage oil is unlikely to solve the underlying issues. Consider a professional inspection instead.
