Castrol GTX 20-50

Bill Crowell said
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.

That may be, but I personally get rid of that light oil as soon as I can. I run 10W-30 in my Dodge Hemi's. That is as light an oil as I will go. All three still run like new. The 2001 Jeep Cherokee and the 1996 Neon have had a life of straight 30 or 20W-50 in the winter. All well past the warranty period and as above, still run like new. Not sure what the motive is for using light oil. Saving Gas? I'd rather my engines last as long as possible.