i use the same weight usually, they have this synthetic oil called elf total 9000 or something...which is supposed to be really good according to the forums. i usually buy 2 liters and 2 filters at a time because shipping is free for orders over $50 and it's like $85ish i think. about the same as you're paying now probably...
personally i like castrol syntec, i run it with that same mobil1 filter in my boosted db8 with no issues. i think the main thing is that oversized filter for increased oil capacity...which would probably help your car for the mountain driving too...