Sorry, I don't buy it. The quality of an oil is determined by a number of measurable qualities. The more robust protection an oil offers (high viscosity at high heat and loads) the better it is, period. I am going on 17 years with one of my S-Types using Mobile One and it remains mechanically excellent, still using not a drop of oil between changes. Obviously, your timetable leaves much to be desired.
BTW, I believe Jaguar also specifies 10K change intervals - twice as long as I'd ever consider doing in Arizona.