Here's an interesting little tidbit: If you compare the MSDS for Startup and GTX High Mileage you will find them IDENTICAL with one exception: the HM oil generally has higher levels of VI improvers (which is not surprising given that the HM oils are blended thicker). Bottom line: all the "good stuff" that's in Startup is also in GTX High Mileage, plus the HM is cheaper.
However, I suspect that the actual percentage of esters used in the Startup is higher than in the HM oil. Castrol could market this oil as a synthetic blend. I guess they choose not to so it won't conflict with Syntec Blend.