Just a comment that I used to follow the same strategy; expensive oil and long intervals. I even did the bypass filter systems.Using amsoil I feel confident taking it to the factory 7500 miles between changes. The oil itself has almost 5k on it and is still very clean and has good viscosity.
Later I did the math and decided I would go the route of cheaper oil -but it has to meet spec AKA Dexos here- and change at shorter intervals.
My thinking today is contamination levels rise over time, regardless of the lube in use. Based on used oil analysis.
Does either choice make a significant difference in engine life? Not so far as I can tell.