Castrol GTX Professional is one of the world's most trusted motor oils. It's more than just oil. It's liquid engineering that protects against problems, like sludge, that your engine faces every day. Severe driving conditions such as every day stop-and-go/stop-start traffic, harsh weather conditions and extended drain intervals, can all cause the build-up of a thick, tar-like substance called sludge. You can't see this sludge build-up but it can damage your engine's performance and cause premature engine failure.