Castrol GTX is one of the worlds most trusted engine oils. It's more than just oil. Its liquid engineering that protects against the problems your engine faces every day. Severe driving conditions can cause the build up of a thick, tar-like substance called sludge. Sludge can block your engines vital oilways like cholesterol - and if untreated it can rob an engine of power and, ultimately, its life. Castrol GTX Double Action formula cleans away old sludge, and protects against new sludge formation better than tough industry standards