Castrol GTX High Mileage is the premium part-synthetic motor oil specially formulated for vehicles with over 75,000 miles. As vehicles age, they change, and some of these changes can result in costly repairs or replacements. Catalytic converter failure is one of the most common and costliest problems faced by higher mileage vehicles.All motor oil contains phosphorus to prevent engine wear. Phosphorus is an industry-recognized catalyst poison. Castrol GTX High Mileage is specially formulated with less phosphorus than other leading oils, replacing it with advanced technology invented and patented by Castrol that helps prevent catalytic converter and emission system failure. It delivers unsurpassed protection against sludge, wear, and oil burn-off, ensuring your engine remains in peak condition even as your vehicle ages.