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 5w30 weight). Perhaps that is the reason the GTX seems to perform better than their other products.