Castrol GTX is one of the world's most trusted engine oils. Environmental factors such as poor fuel quality, hot weather and stop-start driving conditions can lead to sludge, varnish and carbon build-up in your engine: three factors that reduce engine life, power and fuel economy. Castrol GTX is a synthetic blend engine oil. Its double-action formula has been proven to clean away the sludge that can block engine oilways and helps prevent new sludge formation thanks to its synthetic technology. In fact, it provides up to 45 better sludge protection than is demanded by the latest industry standard. If keeping your engine clean is important to you, it's good to know that no other oil keeps engines cleaner.