Castrol GTX 5W-20 is a synthetic blend engine 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 caused by environmental factors such as poor fuel quality hot weather and stop-start driving conditions and reduces engine life power and fuel economy. If keeping your engine clean is important to you Castrol GTX synthetic blend is the oil for you.