Synthetic In High Mile Engine?

As a hobbyist, I have done quite a bit of homework on the entire synthetic/non-synthetic argument. A couple of things to understand is that the molecular structure of synthetic oil, is tiny, little, uniform balls. The molecular structure of crude based oils consist of squares, diamonds, triangles, etc, thus the synthetic molecules are much smaller, leading to their ability to leak were others do not. Having said that, the tiny structure also lends well to better lubrication, reportedly anyways.
As far as viscosity, I think your choice unless specifically needed is a little heavy. I used Mobile 1 in both my daily drivers and have for years. I still insist on regular changes, even though the oil may not necessarily be breaking down, it still gets dirty and there is only so much and for so long that filters can last.
The debate between the two could go on forever with rock-solid arguemtns for either side, the choice is yours.