I've been reading and learning for about 2 months now and I have to say thanks for the education! I've also searched but can never seem to find a discussion that addresses this. I've used GTX forever and always notice that the oil begins to darken almost right away. By the time it gets to 3-4K, it's black, however, wiping the dipstick leaves a dark brown oil stain, so I know it's not really black. Is it soot? How does this affect the performance of the oil? Should I be concerned? Any help will be appreciated.