Castrol GTX High Mileage 10w-30 helps prevent emission system failure. 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 replacement. 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 and delivers unsurpassed protection against sludge, wear and oil burn-off.