Keep changing it every 6K miles until you sell the car. What is the cost/benefit of changing it at larger intervals? If you're doing it yourself, your cost is like $32 each oil change. Why push when the cost is minimal?
I think most people on here, do use a good brand of synthetic. I know Pennzoil Platinum is quite popular nowadays, but Mobil 1 is just as good for what you're doing.
This is my first Boxer engine, but I can't imagine it requires oil changes more often than the competition.
Just follow the manual and you'll be fine.