Castrol® GTX® 5W-20 is a synthetic blend motor oil with a double-action formula proven to clean away the sludge that can block engine oilways* and help prevent new sludge formation. 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. * When tested in engines with a high level of sludge