Patman, I am one of the bigger supporters of Castrol GTX, but I regard it as a short oil drain oil, in the 4000-5000 mile max. . Your test sounds very interesting though, anxious to see the results. One thing I noticed about the Castrol Products is that as you increase into their more expensive products the initial viscosity decreases. Their GTX oil is a good bit higher in initial viscosity than their Syntec Blend and full Syntec products (referring to the 5w-30 weight). Perhaps that is the reason the GTX seems to perform better than their other products.