Castrol GTX 20-50

Re-stating what MidnightSwinger said, just remember that if your engine is still covered under the manufacturer's warranty, you've got to use the grade and weight of oil specified by that manufacturer or your warranty will be void. If you have a newer engine that is supposed to use 0W-20 oil, for example, I have heard of cases where there was insufficient oil flow when the owner tried using heavier oil, and the pistons scored the cylinders. The manufacturer would not honor the warranty under the circumstances.