Castrol GTX High Mileage motor oil is the #1 consumer high-mileage brand. It's a synthetic blend engine oil that helps extend the life of your engine by protecting against sludge, wear and burn-off and also liquid engineered to protect your emission system. One of the leading causes of 'check engine' lights coming on in high mileage cars is actually catalytic converter failure, which can be caused by phosphorous in engine oil. While phosphorous is present in all engine oils for its ability to reduce wear, it is effectively poisonous to catalytic converters. Castrol GTX High Mileage Motor Oil is liquid engineered with Phosphorous Replacement Technology to replace some of the phosphorous with an advanced additive that maintains robust wear protection. When you want to extend the life of your high mileage engine and emission system and find out how many more miles you can go, there is no better choice than Castrol GTX High Mileage Motor Oil.