Castrol® GTX® 20W-50 is a premium conventional motor oil that has been helping to extend engine life for over 50 years. It is specially engineered to prevent harmful build-up of sludge while protecting against new oil splatter formation better than tough industry standards. Sludge is a thick tar like substance caused by severe driving conditions such as heavy traffic poor fuel quality harsh driving conditions and exceeding oil change intervals. Sludge can block your engine s vital oilways like cholesterol and if untreated can rob an engine of its power and ultimately its life.